DEVELOPMENT OF ORIGINAL OPTICAL AND

QUANTUM ELECTRONICS DEVICES FOR

APPLICATIONS IN COMMUNICATIONS,

METROLOGY AND SCIENCES

Hristo Kisov, Margarita Deneva

Elena Stoykova, Marin Nenchev

Technical University of Sofia, Branch Plovdiv, Bulgaria

mdeneva@yahoo.com

Keywords: Interference wedged structures, WDM-system, multi-wavelength lasers, optical transistor, laser with fixed

wavelength at atomic absorption line, injection-locking linear laser amplifier.

Abstract: The goal of the report is to present the development – principles, theories and computer simulations,

experiments and practical realizations, of original and competitive methods, elements and devices for

quantum electronics, optical communications, metrology, remote sensing and sciences: multi-channel

WDM system with independent tuning of each input/output, multi-wavelength laser with independent

control of each wavelength, lasers with emission, spectrally fixed at reference atomic absorption line,

injection-locking laser system for high (~10

-10

) and linear amplification of low-power (~ µW, nW)

modulated laser light, optical analogue of the transistor action – optical transistor, system for remote (up

to kilometres) measurement of small (mm) translational elongation – shrinking of objects, new solution of

tunable sub-nanosecond lasers and lasers with rectangular nanosecond (~1 ns) pulse emission, including

controlled duration and tunable wavelength. The basic element of the devices developed is stable and

compact interference wedged structures in new composite solution with very narrow transmission

(≤ 0.01 nm) in relatively large spectral range (≥1 nm). The laser active media used are solid-state,

semiconductor and dye.

1 INTRODUCTION

The report is a review of our recent results,

concerning the further development of original

quantum electronics and optical devices with a

potential for competitive applications in optical

communications, metrology, scientific work, and

atmospheric pollution monitoring. The aim of the

report is to present as a whole complex our last

achievements in the development – experiments,

theories, and practical realizations of an original

WDM system with independent tuning of each input

and output, system for remote (to kilometres)

measurement of small (mm) translational elongation

- shrinking of objects, optical transistor, multi-

wavelength lasers with independent control of each

wavelength and with wavelength emission in single

beam or in closely parallel or coaxial beams, the new

solution of tunable sub-nanosecond lasers, the lasers

with emission, spectrally fixed at reference atomic

absorption line. The basic elements, used and studied

by us are a wedged interference structure (variation

of the Fizeau Wedge), including some researched of

found by us new properties. The essential part of the

results is obtained in our group at the Technical

University of Sofia, Branch Plovdiv and the

University CNAM-Paris, the University of North

Paris and the University “Sent Quentin” – Versailles,

France. The principal authors’ publications from the

last years, where the discussed here results are given

in details, are the firs 13 articles, given in the

References at the end of the paper. The authors of

the report, that are the main co-authors of the noted

works, have selected and systematized the materials;

also the essential part is based on their propositions –

primarily given in their patents and previous articles

followed the noted firs ones, except the last well

known laser book [Svelto,1998]. The report includes

also completely new, non-published results. The

articles of the other authors, related with the subject

of the presentation, are given as citation in the noted

146

Kisov H., Deneva M., Stoykova E. and Nenchev M.

DEVELOPMENT OF ORIGINAL OPTICAL AND QUANTUM ELECTRONICS DEVICES FOR APPLICATIONS IN COMMUNICATIONS, METROLOGY AND SCIENCES.

DOI: 10.5220/0005415201460157

In Proceedings of the First International Conference on Telecommunications and Remote Sensing (ICTRS 2012), pages 146-157

ISBN: 978-989-8565-28-0

authors’ publications. Following the actual authors’

professional activity, the report concerns mainly the

technical aspects of the problems. Nevertheless, the

necessary physical moments to clarify the principle

of the developed methods and devices are also given

shortly.

The general objective of the work is to establish

these new and effective devices in science and

practice as novel components of the main hardware

basis in the indicated areas. In parallell, new

knowledge in the field of quantum electronics and

optical interferometry is obtained. The envisaged

solutions have encountered their preliminary

positive approbation firstly in the working

laboratory models as well as in the cited below

recent publications in the specialized journals.

2 NEW ELEMENTS AND DEVICES

IN ACTUAL DEVELOPMENT

2.1 Original optical elements based on

wedged interference structures and

their applications

Furstly, we will present the further development and

examine thoroughly the original optical elements

and system for noted in the introduction

applications. Proposed devices are based on the use

of the wedged interference structure of Fizeau type

Interferometer - Interference Wedge (IW). The

development include ideas, experiments, theory and

proposed applications developed by us. It is

applicable in the case of limited diameter beam

illumination [Nenchev, 1982; Stoykova, 2010;

Nenchev, 1993; Stoykova, 1993; Deneva, 2007;

Deneva, 1996]. The new proposition, except our

previosly introduced new optical element based on a

Reflecting Interference Wedge [Nenchev, 1982],

includes a new WDM (wavelength division

multiplexing) element with an important property

allowing spectral tuning of inputs/outputs in the

simplest manner. The importance of WDM

structures for the optical communications is

undisputable and is described in the most popular

textbooks. Also, we describe the new system with

IW for remote (meters, kilometers) measurement of

the translational expansion and shrinking of objects.

The principle of our proposed WDM structure

can be clarified with the Figures given below. The

interference wedge (IW) of thickness of the order of

micrometers plays the role of a spectrally selective

filter and a channel coupler, being a near totally

reflecting mirror for the non-resonant wavelengths.

The new, generalizing theoretical and experimental

physical treatment of the IW is described in our cited

papers [Stoykova 2010; Nenchev 1993; Stoykova

1996].

The Fizeau interferometer or interferential wedge

(IW) consists of two reflecting plane surfaces

separated by a gap with linearly increasing

thickness. A low-reflectivity coated wedge has been

used before us as un effective tool in surface

topography, as a high-resolution broadband wave-

meter. Multiple-beam interference in IW has been

addressed for implementation of phase-shifting

Fizeau interference microscopy. As a rule,

theoretical and experimental analysis of the IW

properties by the other authors were conducted

mainly for the case of infinite plane-wave

illumination assuming an extended wedged

structure; different wedge applications have been

also analyzed for this particular case. Inspired by

IW incorporation in laser design, over the recent

years we focused our efforts on the study of compact

wedged interference structures under illumination

with a narrow light beam of small diameter. We

have succeeded to reveal unique properties of the

IW when illuminated with laser light, and to propose

various applications, thus insuring the IW as a

competitive optical element in laser resonator design

[Deneva, 2007; Louyer, 2003; Stoykova, 1996;

Goris-Neveux, 1995]. A high-reflectivity coating IW

with thickness of tens of millimetres has spectral

resolution comparable to that of the Fabry-Perot

interferometer, but at additional advantage of linear

spectral tuning by means of translation in its plane.

Such IW, with apex angle of 5–100 μrad and

thickness of 5–500 μm has been used by us in laser

resonators technique to create two-channel

resonators with independently controlled

characteristics of the produced two-wavelength

emission, as a spectrally selective reflecting or

transmitting optical element of resonant wavelength

easily adjustable by translation of the interferometer.

Selective wedge transmission and reflection has

been used for continuous tuning of the selected

mode in high-purity single mode lasers,

unidirectional lasing in ring resonators, two-

wavelength laser operation, narrow line selection

and tuning of wide-gain lasers.

A simple and effective solution of a multi-

wavelength spectrally selective resonant structure

for two-wavelength lasers with independent tuning

at each wavelength has been proposed by the authors

[Nenchev, 1981; Deneva, 2007; Louyer, 2003,

Development of Original Optical and Quantum Electronics Devices for Applications in Communications,

Metrology and Sciences

147

Gorris-Neveux, 1995]. It makes the use of valuable

optical features of the IWs. In particular, we

advanced a reflecting IW as a completely new laser

spectral selector. The proposed structure has been

patented and successfully applied in multi-

wavelength laser technology for dye, Ti:Sapphire

Yb:YAG, and semiconductor lasers.

Multiplexing operation Division operation

Figure 1. Schematic presentation of new WDM structure.

Generally speaking, it could be considered as a new

and competitive solution of a WDM element for

optical communications.

The high-reflectivity coatings IW of thickness of

5-300 μm acts simultaneously as a spectrally

selective filter and a channel coupler, being a nearly

totally reflecting mirror for the off-resonant

wavelengths. In addition, tuning of the transmission

maxima is provided by simple translation of the IW

in its plane. Figure 1 represents schematically the

operation of the proposed new WDM structure for

the cases of multi-wavelength input or output beam.

The tuning of each output by translation of the

corresponding IW in its plane does not affect the

geometry of the structure and the characteristics of

the other outputs respectively. Also, as it is clear

from the figure, the structure gives the possibility to

obtain superposition of individual beams which have

passed through each IW, thus serving as a multi-

beam multiplexing element for the spectrally

different beams (each of them being a resonant beam

for the corresponding IW and off-resonant beam for

the other IWs).

Analysis of the WDM element requires first to

analyze the behaviour of a separate IW for laser

(coherent) beam illumination.

We adopted the mathematical model for

computer simulation and developed an adequate

description of transmittance and reflection of the IW

for a limited diameter laser (coherent) beam (~1-

1.5 mm). [Nenchev, 1993; Stoykova, 2010] Thus,

our treatment differs essentially from the plane-wave

illumination approach.

The detailed calculations are given in our works

[Stoikova, 2010; Nenchev, 1993; Stoykova, 2001].

The calculations were made for two types of

interference wedges. The first one is a “sandwich

type” IW, formed by sequential layer-by-layer

deposition of a dielectric reflective coating of

reflectivity 0.9 on both sides of the glass plate,

which represents a wedged transparent layer with

optical thickness of 5 μm. The other type of the IW

is the silica wedge with optical thickness of 300 μm

having dielectric layers on both surfaces of equal

reflectivity of 0.9 in a spectral region of ~ 30 nm

around the wavelength of 630 nm. The apex angle of

both wedges is α = 0.05 mrad. The described IWs,

which are of essential interest as high-resolution

spectral selective elements, are simple for

production and use. The “sandwich type” IW is very

convenient for application in the new WDM

structure due to its compactness.

multichannel output beam

… λ

tune λ

multichannel output beam

… λ

tune λ

multichannel output beam

… λ

tune λ

… λ

(μW)

multichannel input beam

tune λ

… λ

(μW)

multichannel input beam

tune λ

… λ

(μW)

multichannel input beam

tune λ

A typical curve, calculated for illumination with

a CW laser Gaussian beam in the region of its flat

front (flat-concave resonator, near the flat output

mirror), is shown in Figure 2. We see that the IW is

a highly transmissive narrow-line filter for the

resonant wavelength and approximately totally

reflecting mirror for the off-resonant wavelength.

Figure 2. Calculated transmission and reflection curves as

a function of the wavelength for a “sandwich”-type IW

(CW laser beam illumination).



As a second step, we have studied wedge

behavior for illumination with short laser pulses,

including the sub-nanosecond light pulses. The

calculations are made at different wavelengths near

~ 0.6 μm. The obtained results are similar to those in

the case of CW beam illumination.



Figure 3. Transmission and reflection for pulse

illumination (0.5 ns pulse duration); e=5 μm.

First International Conference on Telecommunications and Remote Sensing

148

Figure 3 gives the typical computed curves for

the “sandwich type” IW irradiated with pulses of

duration of 0.5 ns (the axis x in Figure 3 gives the

distance along the beam impact area from an

arbitrary chosen zero-point) [ Nenchev, 2011].

As it can be seen from the depicted curves, the

transmission and reflection properties of the wedge

do not change essentially. Transmission reaches

about 60 % for the pulse duration of ~ 0.1 ns that

may correspond to frequency repetition rate of ~ 10

GHz. Therefore, the presented calculation shows

feasibility of the proposed WDM structure.

We realized experimentally our WDM structure

for the case of CW beam illumination using a

laboratory model of free-communication system. We

formed a single laser beam by exact superposition of

the emissions of three CW lasers – two He-Ne lasers

emitting at 0.63 μm and at 0.59 μm respectively and

the frequency doubled Nd:YAG laser (0.53 μm).

The beam diameter was ~ 1 mm. Figure 4(a)

presents the WDM structure, composed of 3

“sandwich type” wedges with thickness e = 5 μm,

each of them tuned to one of the wavelengths in the

green, yellow and red spectral regions, respectively.

Figure 4(a). The experimental set-up presents the realized

WDM device, composed of 3 wedges, each of them to a

separate channel – green, yellow and red, respectively.

Figure 4(b). Visualization of the channel-separation

(colors) by the new WDM-arrangement in the laboratory

model

As it is shown in Figure 4(b) by using smoke

visualization and screens, wavelengths separation

has been achieved. By translation of the wedges, we

can independently tune the resonance and the given

output. The laboratory WDM is shown in Figure 5.

Figure 5.

Realization of the

compact working

experimental model

of the proposed

WDM structure

The calculation shows the same type of

dependences for the silica gap IW with optical

thickness of 300 µm (Figure 6(a) ) .

The essential advantage of this silica gap thick IW is

the higher spectral resolution in comparison with the

“sandwich type” structure whose thickness is

technologically limited to few µm, and respectively

does not permit to obtain a transmission line low

than few part of nanometers. However, there is the

problem related of obtaining a selection by the

standard IW structure of a narrow line in

combination with high separation between the

resonant lines (Figure 6(a)). The calculations show

that there are completely similar dependence

between the width of the selected line δλ and the

spectral distance ∆λ between the lines as this one for

FPI - e.g. δλ = ∆λ/F, where F is the fines factor,

depending on R. Thus the desired low value of δλ

leads to low value of ∆λ .This limits the selectivity

of the channels in optical communication system.

The principle of our solution of this problem is

based on the use of composite wedged interference

structure. It can be understood from Figure 6(b)

where is given schematically one example of

composed two-component structure.

Figure 6(a). Calculated curves as in Figure 3, but for thick

silica-gap 200 μm IW.

Incident beam – λ

, λ

selected λ

screen

Residual beam at λ

, λ

screen

Incident beam – λ

, λ

selected λ

screen

Residual beam at λ

, λ

screen

Development of Original Optical and Quantum Electronics Devices for Applications in Communications,

Metrology and Sciences

149

Figure 6(b). Schematic of the new composite wedged

interference structure.

It consists of one thick wedge e.g. e

=200 µm

optical thickness silica glass wedge with two

dielectric mirrors at each wedge plane with

reflectivity of R=0.95%. The wedge angle α

of the

plate in the example is α

= 200 µrad. On the one of

the mirrors is lay a transparent wedged layer with

thickness e

=10 µm and wedge angle α

. The

relatively simple calculations give that if the angle

and thickness e

, and the angle α

and the

thickness e

are chosen to be in the relation

= α

/ e

the change of the resonant maximums of both

connected wedges with the translation of the

composite system in its plane is exactly equal. In

this system the thick wedge gives a very low

spectral width of the transmission of all system

(~0.05 nm) and the thin wedge selects only single

resonance of the thick wedge at high spectral range

(~ 15 nm and higher). Typical example of computer

calculated resonances at the described system is

given in Figure 6(c).

Figure 6(c). Computed transmission of the composite

wedged structure, formed by two IW (10 μm and 200 μm)

with convenient wedge angles and the tuning (see the text)

As a second task, we have proposed new and

attractive utilization of the IW specific properties by

designing devices, which allow distant (from few

meters to millimeters) laser measurement of small

(~mm) linear translation of a rigid object.

The principle of our device can be understood

from Figure 7, which shows as an example -

measurement of small linear stretching of a steel

hammer-beam due to change of the IW transmission

resonance in the beam incident point.

The set-up contains a comparative system of one

beam-splitter and two photo-receivers. One of them

records that part of the emitted beam, which forms

reference intensity and the other one records the

light transmitted by the IW. Due to translation of the

IW, the transmitted light decreases. By tuning the

laser, we can obtain new resonant wavelength, λ

corresponding to the new wedge thickness.

Indication for this is the new peak in the transmitted

light that is recorded by the corresponding receiver.

From λ

and λ

1 2

it is easy to calculate the translation

distance ∆x, of the invar plate. Figure 8 proves the

high sensitivity of the proposed method. For high

precision measurements of translations in sub-

millimeter region, a single-mode semiconductor,

Titanium Sapphire or dye lasers can be used.

Figure 7. Device based on IW for distant measurement of

small translation of a rigid object in its plane

642 643 644 645 646 647 648

100

200 μm

10 μm

642 643 644 645 646 647 648

100

, %

For the case of remote (meters, kilometers)

measurement of the translational stretching -

contraction of objects the second type of system,

which is other variant of the idea, discussed above,

is developed. This system eliminates the increasing

of the diameter of the laser beam due to the natural

divergence, also the fluctuation of the illuminated

intensity and the need of exact beam direction on

Figure 8. Calculated

dependence of the

resonance wavelength

at different points

along the IW.

First International Conference on Telecommunications and Remote Sensing

150

the Interference Wedge (which is a small dimension

element). The action of the system is clarified from

given Figure 9.

Here, we introduce an Ulbricht

sphere and lens, as it is shown in Figure 9, to

eliminate the noted above problems for remote

measurement at long distance. The radio-transmitter

system or reflection of modulated by information

about translation part of the incident light transmit

the two signals in the processing system. Thus, we

obtain the correct relative value of the transmitted

signal, what eliminate the influences of the beam

intensity fluctuations and the incident place of laser-

beam cross-section, illuminating the lens.

Figure 9. Device for remote (kilometers) measurement of

small translations of a rigid object in its plane that uses

IW and Ulbricht sphere.

The developed laser-Interference wedge devices can

be of interest to control the metal or concrete

hammer-beam length variation (with the temperature

or earthquake) of bridges, of platforms for oil

extraction in the sea, of walls of the buildings etc.

2.2. Original multi-wavelength lasers

with independent control of each

wavelength using the WDM

structures developed in 2.1.

An important goal is the achievement of two- and

multi-wavelength generation of nano- and sub-

nanosecond pulses with implementation of our

group’s original methodology as well as

development of multi wavelength generators of the

same type based on semiconductor active media

[Nenchev, 1995].

Our group has substantial expertise in the

development of two-wavelength lasers [Deneva,

2010; Louyer, 2003; Slavov, 1998, Gorris-Neveux,

1995; Nenchev, 1981]. Using our original approach

we were the first in the world to develop with the

corresponding theoretical and experimental

background two-wavelength F-centers, Ti-Sapphire

(in pulsed mode) and Yb:YAG lasers, (in a CW

diode pumped mode) – [Loyer, 2003; Goris Neveux,

1995]). The laser solutions are based on our

proposed effective multichannel resonator with a

complex selector-coupler structure based of IWs

[Nenchev, 1981]. Using the described above multi-

channel WDM element new and simple solution of

multi-wavelength lasers are proposed with

independent tuning of each wavelength. Two types

of solution are given in Figure 10(a) and 10(b). The

scheme in Figure 10(a) is solution in which the

emission of the two wavelengths is in single laser

output beam. For many cases where single volume

needs to be illuminated exactly (e.g. in remote

atmosphere pollution control) and high speed

processes (e.g. explosion) this solution is

advantageous. The difficulty for multi-wavelength

operation is related to strong wavelength

competition effects in homogeneously broadened

active medium – e.g. dye, semiconductors. It follows

that it is necessary to make a very precise balance

for net gain for all wavelengths at each tuning or

strongly limit the tuning range around the gain

maximum – in its flat part. The second scheme with

closely spaced parallel beams at each wavelength –

in Figure 10(b) - eliminates the problem of

competition, however the laser light at the separated

wavelengths acts upon different parts of the

illuminated volume (superposition can be obtained

after focusing in small length of ~ mm).

Figure10. WDM multi-wavelength laser resonator

schemes with independent tuning at each wavelength; (a)

–with output in single beam, (b) – with closely parallel

outputs.

Except the previously realized, used and

described in the specialized literature dye, Ti-

Sapphire, F-colour centres and Yb:YAG lasers, in

our recent works we have practically developed a

two-wavelength semiconductor laser. The laser

emits at two independently tunable wavelengths in a

single beam. To obtain a small diameter of the

Development of Original Optical and Quantum Electronics Devices for Applications in Communications,

Metrology and Sciences

151

incident beam at the selected IWs, we modified the

scheme in Figure 10(a) using the focussing and the

flat end mirrors as it is shown in the Figure 11. We

have realised (Figure 12) such two-wavelength

semiconductor laser using a red laser diode with

antireflection coated output surface. The laser

Figure 11. Schematic diagram of the modified resonator,

given in Figure 10(a) and adapted for two-wavelength

generation with independent tuning of each wavelength in

large beam semiconductor active media.

operates successfully at two wavelengths. As a rule

the lasing starts firstly in one of the channels. To

obtain also lasing in the other channel we slowly

increased the losses for the started generation, in

practice by misalignment of the end mirror in its

channel. This can be obtained if the wavelength is

spectrally near the maximum of the gain. At each

tuning of one wavelength it was necessary to arrange

again the losses at the generated channel to obtain

the lasing also at the second wavelength. In this

manner tuning range of ~ 4 nm for each wavelength

in two-wavelength operation can be achieved. By

oscilloscope studies we found that both wavelengths

are generated simultaneously. This laser can be

useful in some experimental works needed in two-

wavelength operation. Our next work is related to

realization of the second, wavelength competition

less, scheme given in Figure 10(b). The new solution

is combined with passive self-injection [Keller,

2000] what abruptly increases the laser efficiency.

Figure 12. Photograph of the operating two-wavelength

semiconductor red laser. The generation is at two

wavelengths λ

and λ

in separated reference outputs

and in main, common output

2.3 Actual our development of a high-

power two-wavelength wavelengths

competition-less Nd:YAG laser

Earlier [Nenchev, 1978], we have patented a

flashlamp pumped laser where single active element

operates in two parts separately and at two different

wavelength. Actually, we have developed this

technique using single, flash-lamp pumped (~150 J

pump) Nd:YAG crystal to obtain simultaneous or in

controlled manner generation at two chosen lasing

tune λ

low-power signal

output at λ

low-power signal

output at λ

laser output

+ λ

AR front face

diode-laser

tune λ

low-power signal

output at λ

low-power signal

output at λ

laser output

+ λ

AR front face

diode-laser

Figure 13(a). Schematic of the two-wavelength, flashlamp

pumped, Q-switched, laser that uses two separate parts of

a single Nd:YAG crystal and prisms selective structure.

lines - pair-combination from the possible

generating lines: 1.06 µm (to 0.8 J), 1.32 µm, 1.34

µm, 1.36 µm (to ~ 0.14 J) and 1.44 µm (to ~0.03 mJ,

none well reproducible) and avoiding the limiting

wavelength-competition effect. The laser (Figure

13(a)) can also operate at two chosen modes at

different frequency distance from the standard for

single cavity lasers (c/2L). We use the generations in

two separate parts of the single, flashlamp pumped

Nd:YAG crystal in two manner: i) in coaxial

separation and ii) in two closely spaced parts by

prism selected-tuning resonators (for the single-

mode case with introduced glass-plate Fabry-Perot

interferometers).

The design of this laser was developed both

theoretically and with practical realization, both for

free lasing and Q-switched regime. To generate any

First International Conference on Telecommunications and Remote Sensing

152

desired pair of the given lines we employ a rotated

prism (axis in the plane or perpendicular) Q-switcher

that is completely non-sensitive of the different

wavelengths. The scheme of experimental

realization of the described two-line and two-mode

laser is shown in Figure 13. The typical oscilloscope

traces of simultaneous generation at two markedly

different lines – 1.06 µm and 1.36 µm, obtained for

conveniently chosen parameters of both resonators

and lasing volumes are shown in Figure 13(b).

Figure 13(b). Simultaneous generation at 1.06 µm and

1.36 µm in free-running regime (for conveniently chosen

parameters).

For the theoretical study we have adapted the

rate differential equations system [Svelto, 1998] to

obtain the optimal conditions for desired operation.

The theoretical considerations show the

possibility to control the energy, time length of the

pulses and delay between them, including also the

possibility for simultaneous Q-switching operation -

by conveniently chosen resonator parameters and

parts of the lasing volumes. The experimental results

are in agreement with the theory. As example in

Figure 14 are presented the oscilloscope traces of Q-

switched generation at the 1.06 µm and 1.36 µm

(outputs ~1 MW).

Figure 14. Experimental curves of temporal tuning of two

wavelengths (the existence of a combination of parameters

permitting simultaneous generation can be seen)

Note, that our rotating prism Q-switcher is very

convenient for described operation due to its

completely independence of the wavelength and its

simplicity of operation.

The advantages of a developed laser in

comparison with the system of two separated and

coupled Nd:YAG lasers are: i) simple construction

ii) essentially low cost and iii) increased efficiency

due to the pumping of single active road

Such line-tunable and two-wavelength laser

devices are of interest for applications in metrology,

wavelength testing and study of non-linear effects in

optical fibre, remote sensing and scientific works.

2.4 Generation of sub-nanosecond

pulses implementing the original

methodology with two-channel

WDM-system based optical

resonator

In detail, the original approach for realizing sub-

nanosecond tunable laser is presented in our paper

[Deneva, 2007]. The essence of the principle is to

restrict the starting pulse-like generation with sub-

nanosecond pulsations (~0.1 – 0.2 ns, “spikes”) to

single pulsation by using an active mirror, which

forces damping the competitive generation in the

second selective channel. Theoretical analysis and

experimental test showed the improvement of the

shape and shortening the duration of the sub-

nanosecond pulse by using our technique in

comparison with known methods. The principle is

clarified by the scheme shown in Figure 15. The

typical pulses obtained by known techniques for

separation of a single spike (Figure 16, left) and by

our proposed technique (Figure 16, right)

demonstrate the advantage of our approach.

Figure 15. Set-up for selection of a single sub-nanosecond

pulsation by active mirror in two-cannel cavity.

Pump

laser

(0.53 μm

harmonic)

PPA

Output 1

(diagrammatically)

Without M

(single channel)

With M

with AMIR

PPB

Active

Mirror

(AMIR)

Output 1

(λ

)

With M

without AMIR

PPI

Output 2

(λ

)

(Q-switched

Nd:YAG)

Pump

laser

(0.53 μm

harmonic)

PPA

Output 1

(diagrammatically)

Without M

(single channel)

With M

with AMIR

PPB

Active

Mirror

(AMIR)

Output 1

(λ

)

With M

without AMIR

PPI

Output 2

(λ

)

(Q-switched

Nd:YAG)

Development of Original Optical and Quantum Electronics Devices for Applications in Communications,

Metrology and Sciences

153

Figure 16. Typical oscilloscope traces (5 ns/div) of the

optimized selected spike: left – for the known technique

of competitive resonators and right- for our proposed

AMIR- approach. Full confirmation of improvement of

the selection by our predicted theoretical approach.

The generated sub-nanosecond pulses are of interest

for testing system in optical communications, in

systems for distance measurements, in scientific

works and in remote sensing.

3 DEVELOPMENT OF LASERS

WITH FIXED EMISSION

FREQUENCY AT REFERENCE

ATOMIC ABSORPTION LINE

A simple, all-optical technique for producing pulsed

semiconductor laser light, spectrally narrowed and

fixed at a chosen absorption atomic line, is realized

and studied by us [Deneva, 2010]. The technique,

which is not of laser locking type, is based on

utilization of a conventional diode laser without any

impact on its operation. For its implementation the

diode laser output is fed to a modified Michelson

interferometer, and controllable disturbing of phase

and amplitude correlation between the interfering

beams in the two arms of the interferometer is

achieved by frequency scanning through a contour

of reference absorption line of substance, introduced

in one of the interferometer arms. Imbalance is

produced by the absorption and the refractive index

changes throughout the contour of the absorption

line. The control of the imbalance is realized by

variation of the optical path length of the other arm

of the interferometer through an appropriate tilting

of a glass plate introduced in this arm.

We have shown both by theory and experiment

that under properly chosen conditions the spectrum

of the obtained light partially overlaps the atomic

line and has linewidth, comparable to the width of

the absorption transition.

The set-up is given in Figure 16. A commercial

single longitudinal mode pulsed diode laser (DBR

type, model SDL-5702-H1) with emission line width

of about 100 MHz was used as a light source. The

wavelength of the selected mode of the diode laser

was repetitively scanned (forward -backward) within

± 10 GHz (~ 0.0210 nm) around the 852.1 nm Cs

absorption line (6S

-6P

1/2 3/2

transition - a single

absorption line within the scanned frequency region.

The scanning was accomplished by the pump current

modulation within ± 5 mA around 44.3 mA. The

line width of the chosen Cs-transition was ≈

0.92 GHz (0.0019 nm, FWHM). The diode

temperature was kept at 17.9

C with accuracy of

±0.1

C. The diode laser beam, after passing through

an optical isolation system (Faraday Isolator or a

combination of a polarizer and a quarter-wave plate,

as shown in Figure 16), impinged the entrance

beam-splitter of a modified Michelson

interferometer composed of the beam-splitter and

wedged full reflecting dielectric mirrors M

and M

1 2

The beams reflected from M

and M

1 2

interfered at

the beam splitter and formed the useful

interferometer output (Output 1 in Figure 17).

A cell with atomic Cs vapour at room

temperature (22

C) was introduced to assure

reference line in the first interferometer’s arm

between the beam-splitter and the mirror M

(at

852.1 nm Cs line).

If the wavelength of the selected mode remained

outside the absorption line during the scanning of

the diode driving current, the interference conditions

did not change, and the Output 1 did not exist.

Figure 18 shows the signals from the diodes PhD

and PhD

; curve A corresponds to the Michelson

interferometer Output 1 whereas the curve B depicts

the signal from the diode PhD

(inverse polarity)

that gives the absorption by the Cs line in the

external reference Cs cell.

Figure 17. Experimental set-up for producing diode laser

light spectrally fixed at the Cs absorption line.

When the laser diode wavelength fell into the

contour of the chosen absorption line, the variation

of the refractive index and the absorption changed

the interference conditions. Thus the destructive

interference was terminated and interferometer

Output 1 appeared.

First International Conference on Telecommunications and Remote Sensing

154

For the optimized conditions (Figure 18(c)),

achieved by appropriate declination of the glass

plate), the locked line is practically a single line with

line width (~1.7 GHz, or 0.0035 nm) that is

comparable to the absorption line width (0.9 GHz,

0.0019 nm) and overlaps the absorption by

approximately 45%. The Output 1 is ~ 2 mW for

~10 mW laser diode emitted power. The

performance is completely reproducible (~ hours).

The theory is in satisfactory agreement with the

experiment and confirms the possibility of such kind

of diode laser light generation.

The reported technique can be useful in a variety

of spectroscopic applications when the target is a

single transition which should be excited to monitor

or separate a particular substance from a mixture of

different substances.

Figure 18. Spectrograms of the laser diode light emitted

from Output 1 (curve A) and of the reference Cs cell

(curve B; inverted signal). The exciting current is scanned.

Another simple system of spectral locking of the

laser emission at the reference absorption line is also

developed [Deneva, 2005; Gacheva, 2008]. Its

principle is based on the disturbance of the

competition between two injection-controlled

generations in a two-channel resonator or two

amplifications in single active medium. The injected

light before the injection in one of the cannels or the

amplifier input passes through a substance with the

desired absorption line. This solution is clarified

from the applied Figure 19.

Figure 19. Frequency locking set-up.

The principle of our injection-locking technique

is theoretically and experimentally demonstrated in

our previous works [Keller, 2000; Slavov, 1998].

4 PRINCIPLE OF NEW

INJECTION-LOCKING

LINEAR AMPLIFIER OF

AMPLITUDE MODULATED

LASER LIGHT

At this point, we describe our principle of new

injection-locking amplifier of amplitude modulated

laser light using counter injection in a ring laser

configuration. The last development includes the

multichannel information laser light amplification

(of the order of ~ 10

and more – from µW to W)

with high linearity of the amplification. To amplify

the injected in ring laser modulated laser light

(simplest practical arrangement of the amplifier) we

introduce counter-injection that compete with the

modulated light and compel the amplification to

follow exactly the modulation. The principle is clear

from Figure 20 [Deneva, 1999].

Pump

active

medium

ccw

output cw

amplified (x10 ) light

output ccw

CW counterinjection

at (~1mW)?

injected beam

1...i

(~W)µ

… λ

(μW)

(~ 1mW)

…

Pump

active

medium

ccw

output cw

amplified (x10 ) light

output ccw

CW counterinjection

at (~1mW)?

injected beam

1...i

(~W)µ

… λ

(μW)

(~ 1mW)

…

Figure 20. A new system for linear amplification based on

injection–locking technique with counter- injection for

linearization of the amplification (possibility – kW output

power, ~ 10

- 10

gain for injected modulated light of

power ~ mW and μW)

We describe the action of the new ring counter-

injection amplifier for the case of multichannel (at 5

wavelength channels) modulated laser beam

amplification by adapting the rate of differential

equations, adding the members that describe

modulated injection and counter-injection.

Figure 21 gives typical calculation for single

sine modulated wave. We have shown the ability of

our amplifier to amplify simultaneously and linearly

a number of injected beams with different frequency

in a large range of ~ 800 GHz. The nonlinear

distortions, defined by the harmonics relative power

are less than 1 %. Such amplification is possible in

very wide range of ~ 2400 GHz. The calculated

Injecting

laser

Pumping

laser

active

medium

absorption

medium at

beam

splitter

beam

splitter

Injecting

laser

Pumping

laser

output

only at

λλ

output

beam

splitter

Injecting

laser

Pumping

laser

active

medium

absorption

medium at

absorption

medium at

beam

splitter

beam

splitter

Injecting

laser

Pumping

laser

output

only at

λλ

output

only at

λλ

output

beam

splitter

output

Development of Original Optical and Quantum Electronics Devices for Applications in Communications,

Metrology and Sciences

155

curves of amplification for 4 wavelengths

(communication channels) are shown in Figure 22.

(a) (b)

Figure 21. Example of operation of our ring-amplifier. (a)

input signal, (b) and (c) – amplification without and with

counter-injection, (d) Fourier spectrum of the (c).

Figure. 22. The curves of

simultaneous amplification of 4

injected lines in the presence of

counter-injection (~1mW). It

can be seen that the

amplification is practically

linear for injected power

variation between 0.01mW and

~ 0.3mW. The amplification

factor is ~ 10

5 DEVELOPMENT OF OPTICAL

TRANSISTOR

The new interferometer type device for light control

by light (DLCL) uses on one hand the high

sensitivity of the Fabry-Perot Interferometer (FPI) or

IW to the losses in the interferometer’s gap [Deneva,

2004]. Our original idea is to use the possibility to

illuminate chosen volumes of the edge of

interferometer’s or wedge’s gap in two quite

different manners: i) through the interferometer

mirrors (beam A- as shown in the figure); ii) directly

into the gap (beam B). If the gap is full with

saturable absorption medium and the mirrors are

high reflective – e.g. 0.92–0.99, the beam A will

affect the saturable absorber transmission only by

transmitted small part through the mirror and

respectively the FPI transmission will be drastically

low for this beam. When the beam B illuminates

directly the saturable absorber the effect of this

illumination is very strong (no decreasing the

illuminated light intensity by the mirror). Thus, with

the low power beam B we can control in efficient

manner (or to open and stop) the FPI or IW

transmissivity for beam A. One first application of

the new optical transistor will be to forms rectangular

nano- and sub-nanosecond pulses as it can be

understand from the Figure 23.

Nd:YA

laser

:YAG

(IFP plate)

IFP mirror

IFP

beam B

beam A

Figure 23. Schematic diagram of a Cr

:YAG- DCLC and

of the experimental set-up for forming controlled duration

rectangular laser light pulse. OR – optical receiver-

synchronizer, PC-Pockel’s cell, GP-Glan Prism, M

1 2

high reflectivity mirrors. The high speed switching PC

(~ 1-2 ns), activated near the maximum of the input ~ 30

ns pulse, switches the polarization and the GP forms two

spatially separated pulses that act in the described manner

upon the Cr

gap FPI or IW.

Table 1: Example of transmissivity of new DLCL for

:YAG as a saturable absorber [Nenchev, 2011]. The

parameters of the DLCL are given in the table.

R of the

mirrors

IFP or IW

Thickness, mm

Illuminating

beam energy

density

J/cm

Controlling

beam energy

dens. J/cm

0.99

0.4 0.5 (0.5) 0 (0.1|) 3(8.6)

0.99

0.2 0.5 (0.5) 0 (0.1| 3( 21)

0..99

0.1 0.5 (0.5) 0 (0.1) 9(40)

0.92

2.65 0.5 (0.5) 0 (0.1) 9(40)

0..52

20 0.5 (0.5) 0 (0.1) 1(10)

First International Conference on Telecommunications and Remote Sensing

156

ACKNOWLEDGEMENTS

The authors thank to our colleagues from University

CNAM-Paris, University of North Paris, University

“Sent Quentin” – Versailles and the University of

South Paris, France for fruitful collaboration. We

thank also to National Science Fund and Technical

University of Sofia, Division R&D for the financial

support, Contract D-RILA 01/7-19/13.12.2011

(

Pr. 25197 VB).

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