Dual-Frequency VECSEL at Telecom Wavelength for Sensing

Applications

Léa Chaccour

1,2

, Guy Aubin

, Kamel Merghem

, Jean-Louis Oudar

, Aghiad Khadour

Patrice Chatellier

and Sophie Bouchoule

IFSTTAR, Laboratoire LISIS, Marne-la-Vallée, France

LPN-CNRS, UPR20, Route de Nozay, 91 460 Marcoussis, France

Keywords: Semiconductor Lasers, Vertical External Cavity Surface Emitting Lasers, Semiconductor Disk Lasers, Dual-

Frequency Laser Emission, Laser Diodes for the Generation of Radio-Frequency Signals, Optical Sources

for Optical Fiber Sensors.

Abstract: We aim at realizing an optically-pumped, dual-frequency VECSEL at telecom wavelength (1.5 µm) with a

frequency difference in the radio-frequency (RF) range (around 11 GHz), to be used in a sensor unit based

on Brillouin scattering in optical fibers. Laser emission of two orthogonally-polarized cavity modes with a

controlled frequency difference is obtained by inserting a birefringent crystal in the VECSEL cavity. We

have examined the influence of the different intra-cavity elements on the laser emission. It is shown that

optimizing the free spectral range and the bandwidth of the intra-cavity Fabry-Perot etalon is of practical

importance to achieve a stable single longitudinal laser emission for each of the two orthogonal

polarizations. The optimization of the output power has also been investigated and it is concluded that up to

100 mW output power can be expected by adjusting the reflectivity of the output coupling mirror of the

VECSEL cavity. The achievement of a highly-stable frequency difference is crucial for sensing

applications. For this reason the influence of different parameters on the stability of the dual-frequency

emission have been studied. It is concluded that mechanical vibrations are the main cause of the RF signal

instability in our free-running VECSEL cavity. The design of a compact or mono-block cavity may allow to

meet the stability requirements for our sensors.

1 INTRODUCTION

For over two decades, distributed optical fiber

sensors based on Brillouin scattering have gained

significant interest for their ability to monitor

temperature and strain in large infrastructures. When

an incident light interacts with acoustic phonons

propagating in the fiber core, Brillouin scattered

light is generated. The Brillouin frequency depends

on temperature and/or strain variation in the optical

fiber. Sensing techniques are based either on

Brillouin spontaneous scattering (Zou, et al., 2015)

or on Brillouin stimulated scattering (Brown, et al.,

1999). In both cases the signal detection is achieved

in the high frequency domain, e.g. at the scattered

Brillouin frequency, υ

~11GHz in single-mode

standard fibers (SMF), where the electronic devices

used for the detection and analysis of the signal are

expensive. A solution was proposed by Geng et al.

to realize the signal detection and analysis at a lower

frequency, helping to get rid of these expensive and

cumbersome electronic devices (Geng, et al., 2007).

This solution consists in using a Brillouin fiber laser

pumped by a fiber laser source. A part of the

incident light emitted from the fiber laser is injected

in the fiber under test, while another part of this

incident signal is used to pump the Brillouin laser

which acts as a local oscillator and has a frequency

difference with the incident laser close to 11 GHz.

The signal detection is realized at the frequency (ν

), where ν

is the beat frequency of the local

oscillator. This is a good solution to reduce the cost

of the detection system, but the synchronization of

the two fiber laser sources is rather complex wich

will raise the cost of the sensor unit.

Based on this idea we suggest the use of a dual-

frequency VECSEL, where the two frequencies (the

frequency used to pump the fiber under test and the

frequency of the local oscillator) share the same

cavity. A stable frequency difference could be

Chaccour, L., Aubin, G., Merghem, K., Oudar, J-L., Khadour, A., Chatellier, P. and Bouchoule, S.

Dual-Frequency VECSEL at Telecom Wavelength for Sensing Applications.

DOI: 10.5220/0005965200530058

In Proceedings of the 13th International Joint Conference on e-Business and Telecommunications (ICETE 2016) - Volume 3: OPTICS, pages 53-58

ISBN: 978-989-758-196-0

expected in such a configuration, and costly

electronics to synchronize the two frequencies may

be avoided. The on-going work for the realization of

a dual-frequency VECSEL emitting at 1.5 µm is

presented in the following.

2 EXPERIMENTAL RESULTS

2.1 Overview of Dual-Frequency

Lasers at 1550Nm

Before 2009, dual-frequency micro-lasers were

based on diode-pumped solid-state lasers, e.g.,

Nd:YAG, Yb,Er:Glass and Yb:KGW (Alouini, et al.,

1998) (Brunel, et al., 1997) lasers. An electro-optic

modulator was used for frequency separation and a

Fabry- Perot (F-P) etalon was used to ensure a single

longitudinal mode operation on each polarization. A

dual-frequency vertical extended cavity surface

emitting laser (VECSEL), emitting at 1µm, was first

reported by Baili et al. (Baili, et al., 2009), to benefit

from the class A operation of this semiconductor

disk laser, which is free from relaxation oscillations.

The cavity concept was the same as for the dual-

frequency crystal-doped solid state lasers. A F-P

etalon was inserted in the extended cavity to select a

single-longitudinal cavity mode, and a birefringent

crystal (YVO

) was used to ensure a frequency

separation between the two orthogonal polarizations.

In this first report, the VECSEL chip was grown on

a GaAs substrate. The bottom highly-reflective

mirror, a distributed Bragg reflector (DBR),

consisted of alternating GaAs and AlGaAs quarter-

wavelength layers, and the active region consisted of

InGaAs/GaAs quantum wells to emit around 1µm.

Based on this idea, in 2012 F.A. Camargo et al. have

demonstrated the first dual-frequency VECSEL

operating at 852nm dedicated to the coherent

population trapping of Cesium atoms (F.A.Camargo,

et al., 2012). The cavity principle was the same (a

birefringent crystal and a Fabry-Perot etalon were

used), but this time the VECSEL active region was

composed of GaAs quantum wells, embedded in

AlGaAs barriers and pump-absorbing layers. In

2014, De et al. (De, et al., 2014) reported the first

dual-frequency VECSEL emitting at 1550nm using

the same cavity configuration. The VECSEL chip

was grown on an InP substrate and InGaAlAs multi

quantum-wells were used as the active region. Our

aim is to realize a dual-frequency VECSEL emitting

at 1550nm with a frequency difference of ~ 11GHz,

close to the scattered Brillouin frequency in SMF.

As the Brillouin sensors use optical fibers, it is

interesting to use a laser source operating at 1550nm

where the optical attenuation is minimum. In order

to integrate this source into a Brillouin fiber sensor

some specifications have to be met:

- Laser output power typically lower than 10mW for

spontaneous Brillouin sensors

- Laser output power of a hundred of mW, for

stimulated Brillouin sensors

- RF linewidth of the beat note < 0.5MHz, with a

temporal jitter/drift typically <0.5 MHz over several

minutes to allow the detection of ~0.5°C

temperature variation (Shimizu et al., 1992).

The realization of the 1.5µm dual-frequency

laser source is detailed in the following. The

achieved results are presented and compared to the

targeted specifications.

2.2 VESCEL Chip Fabrication

Figure 1: VECSEL chip emitting at 1550nm.

The completed VECSEL structure is schematically

depicted in Figure 1. The VECSEL chip is similar to

that used for the fabrication of the first dual-

frequency VECSEL at 1550nm (De, et al., 2014).

Briefly the active region is grown on an InP

substrate. It is designed for optical pumping at

980nm and includes strained InGaAlAs quantum

wells. A 17-pair GaAs/Al

0.97

0.03

As DBR is

integrated to the active region using a metamoprhic

regrowth (Tourrenc, et al.,2008). A gold layer is

deposited on the surface of the Bragg mirror to

enhance its reflectivity. The semiconductor chip

with deposited gold is then integrated to a CVD

diamond host substrate using metallic bonding. The

InP substrate, and the etch-stop layer are then

removed using selective chemical etching. Finally

the InP top layer of the VECSEL may be finely

etched to tune the resonance wavelength of the

microcavity close to 1550nm, and an anti-reflective

(AR) coating at 980 nm is deposited on the surface

to enhance pump absorption (Zhao, et al., 2012).

OPTICS 2016 - International Conference on Optical Communication Systems

2.3 VECSEL Cavity Configuration for

Dual-Frequency Emission

The fabricated VECSEL chip was first qualified in a

simple plane-concave cavity, as depicted in Figure 2.

Figure 2: Simple VECSEL cavity.

The VECSEL is assembled with a highly-

reflective concave mirror (output coupler) to form a

stable plane-concave cavity. The VECSEL chip is

pumped by a 980-nm laser diode at 45° incidence.

The VECSEL output power versus pump power (L-

P curve) obtained with the concave mirror used in

the dual-frequency experiments, and having a

reflectivity of 99.7% is reported in Figure 3.

Figure 3: Output power obtained in the simple cavity of

Fig. 2. The cavity length is of 8.8mm, the concave mirror

has a radius of curvature of 10mm (leading to a cavity

mode radius equal to ω

=40µm).

To realize the dual-frequency laser operation a

birefringent crystal plate (YVO

) cut at 45° of the

optical axis is inserted in the extended cavity. The

plate is AR-coated at 1550nm on both faces. This

type of birefringent crystal leads to both a frequency

separation and a spatial separation (s) of the two

orthogonally-polarized longitudinal cavity modes.

For a plate thickness of 500 µm, s=50µm. An intra-

cavity F-P etalon allows to select only one

longitudinal mode for each of the two polarizations.

In our experiment a glass (SiO

) etalon with a

thickness of 160µm (free spectral range FSR=5nm)

has been used. The cavity scheme is depicted in

Figure 4. The cavity length was fixed at ~ 8.8mm in

order to maintain a cavity free spectral range larger

than 11GHz.

Figure 4: Schematic of the dual-frequency VECSEL

cavity. On the left: schematics of the spatially-separated

cavity modes (blue) and pump spot (brown).

After the insertion of the YVO

crystal, and fine

tuning of both, the pump position and output coupler

position, simultaneous and robust oscillation of the

two orthogonally-polarized eigenstates is achieved,

as illustrated in Figure 5. The optical pumping

system has been designed to create an elliptical

pumping spot with an adapted size to pump

uniformly the two spatially-separated cavity modes

at the VECSEL chip surface.

Figure 5: Optical spectrum of the dual-frequency laser

emission. Cavity length~8.8mm, YVO

(500-µm thick),

SiO

Fabry-Perot etalon (thickness of 160µm).

2.4 Results

2.4.1 Influence of the Intra-Cavity Elements

and Output Coupling Mirror on the

Output Power

We have measured the VECSEL output power

versus incident pump power (L-P curve) in dual-

frequency operation. After the insertion of the intra-

cavity elements the output power was typically

reduced to half, as illustrated in Figure 6. The

maximum output power may be sufficient for a

sensing unit based on spontaneous Brillouin

scattering, but is not sufficient for stimulated

Brillouin scattering.

Dual-Frequency VECSEL at Telecom Wavelength for Sensing Applications

Figure 6: Dual-frequency laser output power versus pump

power using an output coupler of reflectivity 99.7%.

We have therefore examined the influence of the

cavity elements on the cavity losses and laser

efficiency.

Figure 7: VECSEL L-P curve with a 500-µm, and a 1-mm

thick YVO

plate at normal incidence in the cavity, and

with a 500-µm thick YVO

plate rotated by 9° in the

cavity (the output coupler reflectivity is 99.7%).

Figure 7 shows the VECSEL L-P curve with

YVO

plates of different thicknesses and different

orientations in the cavity. A 0° incidence means that

the plate is normal to the cavity axis. Two

conclusions can be made by examining the figure.

Firstly, the thickness of the YVO

plate doesn’t have

any significant impact on the output power, and this

birefringent crystal is therefore well adapted to the

wavelength of 1550nm. Secondly a slight rotation of

the birefringent crystal (by 9° in the

Figure 7

) in the

cavity doesn’t affect significantly the output power.

Figure 8: Influence of the rotation of the intra-cavity

Fabry-Perot etalon on the output power.

On the other hand, also using the same output

coupler, Figure 8 shows that a slight rotation of the

F-P etalon causes high intra-cavity losses.

Finally, Figure 9 shows the effect of the output

coupler reflectivity on the laser efficiency and

maximum output power.

Figure 9: L-P curves obtained for different output coupler

reflectivities.

The L-P curves depicted in Figure 9 were all

obtained for a cavity mode waist close to 40µm.

It can be seen that up to 100-mW output power

can be obtained by changing the reflectivity of the

output coupler. This result indicate that more than 50

mW output power may be expected in dual-

frequency laser operation by replacing our actual

output coupler by another one having a reflectivity

coefficient closer to R3~99%. This value is

compatible with both spontaneous and stimulated

Brillouin scattering.

2.4.2 Influence of the Fabry-Perot Etalon on

the Stability of the Laser Emission

It is crucial to maintain single-longitudinal lasing on

each of the two polarizations in order to obtain a

stable, dual-frequency laser emission. Therefore the

OPTICS 2016 - International Conference on Optical Communication Systems

FSR of the intra-cavity F-P etalon must be large

enough to avoid simultaneous lasing of cavity modes

located at different successive transmission maxima

of the F-P etalon. For this reason, we have replaced

the 160-µm thick SiO

etalon (FSR = 5 nm) by

another one (SiO

, thickness of 50µm and FSR

=13nm). It is observed that it is easier to select a

single cavity mode with a larger FSR of the etalon.

However in this case a dual-frequency emission with

frequencies largely separated (up to 30 GHz) is

generally obtained, and the frequency difference is

not stable as illustrated in Figure 10.

Figure 10: Evolution with time of the optical spectrum of

the dual-frequency laser emission with a 50-µm thick SiO

F-P etalon having a FSR of 13nm.

It can be concluded that an ideal F-P etalon

would have a FSR close to that of the 50-µm thick

SiO

etalon (FSR ~ 13 nm), and a bandwidth close

to that of the 160-µm tick SiO

etalon to ensure

stable dual-frequency laser emission.

2.4.3 Influence of the Birefringent Crystal

on the Stability (through the Mode

Coupling Strength)

By changing the thickness of the birefringent crystal,

the spatial mode separation is affected and the mode

coupling strength is thus modified. Since quantum

wells have a homogeneous gain, a coupling strength

too close to 1 will induce mode competition and

unstable dual-frequency operation (Pal, et al., 2010).

On the other hand widely separated modes (i.e. with

a mode coupling strength near zero) may show

uncorrelated noise and therefore a broader and less

stable frequency difference. We have therefore

investigated the influence of the mode coupling

strength on the dual-frequency emission by changing

the thickness of the YVO

plate. For a fixed cavity

mode size, changing the spatial separation modifies

the mode coupling strength. The optical spectrum of

the dual-frequency emission obtained with a cavity

length of 8.8mm, a 160-µm thick F-P etalon, and a

1-mm thick YVO

plate is reported in Figure 11.

The mode separation is two times larger than with

the 500-µm tick YVO

plate. Figure 11 shows that

stable dual-frequency emission can be obtained. A

similar result is found (not shown) for a 250-µm

thick YVO

plate.

Figure 11: Optical spectrum of the dual frequency

emission with a 1-mm thick YVO

plate (the coupling

strength is lower than the case of a 500-µm thick YVO

plate). The spectral resolution is of 1 GHz.

2.4.4 Influence of the Mechanical Vibrations

on the Stability without Active

Stabilization

Finally we have examined the stability of the beat

frequency in the MHz range. The experimental set-

up is showed in Figure 12.

Figure 12: experimental set-up for measuring the beat

frequency (Pol: Polarizer; OSA: Optical Spectrum

Analyzer; EDFA: Erbium Doped Fiber Amplifier ; PD:

photodiode ; ESA: Electrical Spectrum Analyser).

An optical polarizer with a 45° orientation is

placed at the output of the dual-frequency VECSEL.

The laser light is then injected in a 1x2 coupler, via

an optical isolator. The first arm of the coupler is

connected to the Optical Spectrum Analyser (OSA)

while the second arm is connected to an Electrical

Spectrum Analyser (ESA), for simultaneous

measurement of both spectra.

Dual-Frequency VECSEL at Telecom Wavelength for Sensing Applications

Figure 13: ESA spectrum for cavity length~8.8mm, output

coupler radius of curvature=10mm, YVO4 500µm.

A typical signal corresponding to the frequency

difference is shown in Figure 13. The full width at

half-maximum (FWHM) can be estimated to be of

the order of 200kHz. Dual-frequency VECSELs

operating at 850nm and using a similar cavity

configuration have shown a beat note FWHM of

~150 kHz without any active stabilzation

(A.Camargo, et al., 2012), which is similar to the

above result.

3 CONCLUSIONS

We have fabricated a VECSEL chip for laser

emission at 1550nm, and we have assembled a

VECSEL cavity for dual-frequancy operation at this

wavelength. Stable dual-frequency emission with a

frequency difference of ~11GHz has been obtained.

Our experimental results show that more than 50mW

output power can be expected in dual-frequency

operation, which is compatible with the

specifications of a Brillouin sensor. The

optimization of the intra-cavity elements, namely the

F-P etalon, can help to ensure a long-term stability

of the dual-frequency emission without mode

hoping. In the range explored in this work, the mode

coupling strength has a low impact on the stability of

the dual-frequency emission, allowing to adapt the

cavity mode size to the pump spot size. Presently

mechanical vibrations appear to be the main cause of

the frequency difference instability. Similar dual-

frequency VECSELs operating at other wavelengths

have shown a similar RF signal linewidth without

any active stabilization. As a conclusion, the design

of a compact or mono-block cavity may allow to

meet the stability specifications required for optical

fiber Brillouin sensors.

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OPTICS 2016 - International Conference on Optical Communication Systems