Hi-Bi Sagnac Interferometer Application for Wavelenght Tuning in

CW and Actively Q-switched Erbium Fiber Laser

Manuel Durán-Sánchez

1,2

, Ricardo I. Álvarez-Tamayo

, Olivier Pottiez

, Baldemar Ibarra-Escamilla

Evgeny A. Kuzin

and Antonio Barcelata-Pinzón

Coordinación de Óptica, Insitituto Nacional de Astrofísica, Óptica y Electrónica (INAOE),

L. E. Erro 1, Sta. Ma. Tonantzintla, Puebla 72824, Mexico

Consejo Nacional de Ciencia y Tecnología (CONACyT),

Av. Insurgentes Sur 1582 Col. Crédito Constructor Delegación Benito Juárez, Mexico D.F. 039040, Mexico

Departamento de Óptica, Centro de Investigaciones en Óptica (CIO),

Lomas del Bosque 115 Col. Lomas del Campestre, León, Guanajuato 37150, Mexico

División Mecatrónica, Universidad Tecnológica de Puebla,

Antiguo Camino a la Resurrección 1002-A Zona Industrial Oriente, Puebla 72300, Mexico

Keywords: Fiber Lasers, Fiber Optic Filter, Sagnac Interferometer, Erbium-doped Fiber, Actively Q-switch.

Abstract: An experimental analysis for the use of a Sagnac interferometer with high birefringence fiber in the loop as

generated laser wavelength tuning device in a ring cavity Erbium-doped fiber laser in continuous wave and

pulsed actively Q-switched regimes is presented. The maximal tuning range of ~26.72 nm depends on the

Sagnac interferometer wavelength spectrum period of ~30.32 nm. The wavelength tuning of the generated

laser line is performed by wavelength displacement of Sagnac interferometer spectrum by temperature

variations applied on the fiber loop. Experimental results of the laser spectrum in continuous wave and

actively Q-switched operations, and Q-switched pulses characteristics are shown.

1 INTRODUCTION

Fiber Optical Sagnac interferometers (FOLM) had

been successfully used as optical mirrors on design

and implementation of fiber laser cavities. FOLM

advantages include simple fabrication and design,

low cost and high sensitivity. In particular, the

Sagnac interferometer with high birefringence fiber

in the loop (Hi-Bi FOLM) has been studied as an

element that allows adjustment of the loss within the

laser cavity in dual-wavelength fiber lasers using

fiber Bragg gratings for wavelength selection

(Durán-Sánchez et al., 2010). However, the Hi-Bi

FOLM can also be an effective device for selection

and tuning of the generated laser wavelength in

lasers with single wavelength or multi-wavelength

operation. (Huixtlaca et al., 2008); (González et al.,

2010).

The Hi-Bi FOLM presents a periodical

transmission spectrum that allows the obtaining of a

reflected beam spectral selectivity within the cavity

related with the birefringence and the length of the

fiber loop. The Hi-Bi fiber loop birefringence

change by temperature variations applied on the

fiber loop causes a wavelength displacement of the

Hi-Bi FOLM periodical spectrum. The Hi-Bi FOLM

spectrum amplitude adjustment is achieved by

adjusting the twist of the Hi-Bi fiber segment with

coupler port fiber splices (Kuzin, 1999).

Moreover, the fiber lasers based on the use of

erbium doped fiber (EDF) as a gain medium have

been extensively studied and characterized in

continuous wave (CW) (Ball and Morley, 1992), and

pulsed actively Q-switch technique, (Delgado-Pinar

et al., 2006); (Cuadrado-Laborde et al., 2010). These

previous studies allow the use of the Hi-Bi FOLM

for its experimental study as a generated laser

wavelength tuning device within a laser cavity with

well-known operating characteristics.

Furthermore, actively Q-switched technique

application in fiber lasers for short pulses (in

nanoseconds range) obtaining, have been studied

due to their performance characteristic of higher

energetic pulses obtaining with improved stability

(Durán-Sánchez et al., 2015). With the onset of

acousto-optic modulators (AOM) with fiber

Durán-Sánchez, M., Álvarez-Tamayo, R., Pottiez, O., Ibarra-Escamilla, B., Kuzin, E. and Barcelata-Pinzón, A.

Hi-Bi Sagnac Interferometer Application for Wavelenght Tuning in CW and Actively Q-switched Erbium Fiber Laser.

DOI: 10.5220/0005656402990304

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 301-306

ISBN: 978-989-758-174-8

301

pigtailed connectors, the overall performance of

laser has been improved. Pulsed fiber lasers by the

actively Q-switched technique applications include

LIDAR systems, medicine, optical fiber sensing,

teraheartz generation, among others. Many of these

applications require efficient pulsed laser emission

with generated laser wavelength tuning over a

significantly wide range and simple experimental

setup designs. Reported tunable Q-switched fiber

lasers generally use fiber Bragg gratings (FBG) for

generated laser wavelength selection and tuning

(Zhou et al., 2010); (Ahmad et al., 2013);

(González-Garcia et al., 2015).

In this paper, an experimental analysis of

generated laser wavelength tuning method by using

a Hi-Bi FOLM is presented. The proposed method is

implemented on a ring cavity Erbium-doped fiber

laser operating in CW and actively Q-switched

operation. The use of the Hi-Bi FOLM as a device

for generated laser wavelength tuning represents a

simple, effective, affordable and adaptable method

to the requirements of specific wavelength laser

emission in a wavelength range for fiber laser

applications.

2 EXPERIMENTAL SECTION

2.1 Experimental Setup of

Erbium-doped Fiber Laser

Figure 1 shows the experimental setup for the

proposed fiber laser with CW and actively Q-

switched operation. The ring cavity is based in the

use of 3 m of EDF as a gain medium. The EDF is

pumped with a 60mW laser diode at 980 nm through

a wavelength division multiplexer (WDM).

Figure 1: Proposed ring cavity Erbium-doped fiber laser

experimental setup.

The Hi-Bi FOLM consist of a 50/50 optical

coupler with output ports (ports 3 and 4)

interconnected by ~19 cm of high birefringence (Hi-

Bi) fiber with birefringence of 4.2210

-4

. The Hi-Bi

fiber is placed on a Peltier device which applies

temperature variations driven by an electronic

temperature controller/meter. The cavity is

completed by an optical isolator for beam

propagating direction establishment within the

cavity, an acousto-optic modulator driven by a RF

signal generator for actively Q-switching technique

application, and a 90/10 optical coupler port 4 is

used to measure the laser spectrum by an optical

spectrum analyser (OSA), and for pulses

measurements by a photodetector and a oscilloscope.

2.2 Hi-Bi FOLM Transmission

Spectrum Characterization

The Hi-Bi FOLM acts as a spectral filter with a

periodic transmission spectrum. The wavelength

period is determined by the Hi-Bi fiber loop length,

the signal wavelength and the Hi-Bi fiber

birefringence (Álvarez-Tamayo et al., 2010). With a

Hi-Bi fiber length of 19 cm, the calculated

wavelength period for the Hi-Bi FOLM transmission

spectrum is ~30 nm.

The splices between coupler output ports and Hi-

Bi fiber are placed in rotation stages for Hi-Bi

FOLM transmission spectrum amplitude adjustment

(Kuzin, 1999). The reflection maxima depends on

Hi-Bi fiber birefringence axes orientation (Durán-

Sánchez et al., 2010). With the proper rotation

adjustment on the Hi-Bi fiber with the FOLM output

ports splices, the maximum transmission of the Hi-

Bi is achieved.

Figure 2: Hi-Bi FOLM measured transmission spectrum.

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Figure 2 shows the measured Hi-Bi FOLM

transmission spectrum in the proposed experimental

setup. Measurements were performed with the cavity

opened between the 50/50 coupler and the AOM

connection with low pump power (around 30mW) at

the 50/50 unconnected coupler port (coupler port 2)

with an OSA. As it can be observed, the measured

Hi-Bi FOLM transmission spectrum exhibits a

periodical function with a wavelength period of

~30.32 nm.

The Hi-Bi FOLM transmission spectrum exhibits

wavelength period differences (observable between

each period), attributed to the amplified spontaneous

emission (ASE) of the EDF amplification used as

input signal.

3 RESULTS AND DISCUSSION

3.1 CW Fiber Laser Operation on

Wavelength Tuning

Spectrum measurements of the generated CW laser

tuned wavelengths by Hi-Bi FOLM fiber loop

temperature variations is presented in Figure 3. The

experimental results were measured with a pump

power of 60mW at the 90/10 coupler port 4 (output

port) with an OSA. For the CW laser operation, the

AOM was removed from the experimental setup

shown in Figure 1 by closing the ring cavity between

the 50/50 coupler port 2 with the 90/10 coupler port

1. As it is shown, the generated laser line is tuned in

a wide wavelength range of ~26.72 nm, from

1551.58 nm with Hi-Bi fiber loop temperature of

29.5°C to 1578.3 nm at 11.94°C. The Hi-Bi FOLM

wavelength spectrum period corresponding to the

longer wavelength limit for Erbium amplification

spectrum has preference to generate laser emission.

Figure 3: Measured CW laser line wavelength spectra

tuned by Hi-Bi FOLM fiber loop temperature variations.

As it is observed, the generated laser line

exhibits amplitude variations depending on the tuned

wavelength. This instability is significantly

attenuated by increasing the pump power.

The laser wavelength tuning on the Hi-Bi FOLM

fiber loop temperature variations can be linear fitted

with a slope of approximately -1.47 nm/1°C.

Figure 4 shows the generated laser lines in both

wavelength tuning limits (1551.58 and 1578.30 nm).

As can be observed, in the wavelength tuning limits

a simultaneously generated laser lines occurs at

wavelength separation of ~30.32 nm, corresponding

to the Hi-Bi FOLM transmission spectrum period.

At the tuning limit, another Hi-Bi FOLM

transmission period is competing for the laser

wavelength generation.

The use of FBGs for generated laser wavelength

tuning is based in the mechanical strain application

resulting in the grating physical deformation. This

method usually allows the laser line tuning in a

narrow wavelength range (less than 10nm) due to

the possibility of irreversible damage of the FBG.

Moreover, the proposed laser wavelength tuning

method is performed by optical means. Since it is

not a tuning method by mechanical deformation,

wide tuning wavelength range only depends on the

EDF amplification spectrum wavelength range and

the Hi-Bi FOLM spectrum wavelength period.

Therefore, the maximal laser wavelength tuning

range depends on the Hi-Bi FOLM transmission

spectrum period and is reduced (to 26.72 nm) by the

dual-wavelength laser operation generated in the

limits, which is produced where competition for

lasing of two different periods occurs.

Figure 4: Generated CW laser lines at both wavelength

tuning limits.

3.2 Tunable Actively Q-switched Fiber

Laser Operation

With the experimental characterization of

Hi-Bi Sagnac Interferometer Application for Wavelenght Tuning in CW and Actively Q-switched Erbium Fiber Laser

303

wavelength tuning by temperature variations of Hi-

Bi FOLM fiber loop obtained in CW laser operation,

laser performance with generated wavelength tuning

in pulsed regime by the actively Q-switched

technique is obtained.

Similar results of laser output spectrum

measurements for CW regime (Figure 3) were

obtained in actively Q-switched laser operation for

the experimental setup shown in Figure 1.

Experimental results presented in terms of generated

laser wavelength tuning on Hi-Bi FOLM fiber loop

temperature variations are shown in Figure 5.

Figure 5: Actively Q-switched laser line wavelength

tuning on Hi-Bi FOLM fiber loop temperature variations.

A group of ten measurements of laser line

wavelength tunings are shown in a wavelength range

from 1542 to 1560 nm corresponding to Hi-Bi fiber

loop temperature variations from 22.62 to 35.12°C.

As it is shown, the obtained laser lines are generated

in a different wavelength range in comparison with

the laser lines obtained in CW. With the AOM

including within the cavity, the cavity losses are

adjusted modifying the Hi-Bi FOLM transmission

spectrum amplitude for each wavelength period. As

a result, the generated laser emission is shifted to

another preferred Hi-Bi FOLM spectrum period in a

different wavelength range.

The Q-switched laser wavelength on Hi-Bi

FOLM fiber loop temperature variations can be

linear fitted with a slope of 1.5 nm/1°C. Therefore,

no significant variation in wavelength tuning on loop

temperature slope between CW and Q-switched

laser operations is observed.

Figure 6 shows the pulse profiles for Q-switched

fiber laser operating for different repetition rates.

Measurements were performed at output port with a

photodetector and monitored in an oscilloscope. The

repetition rate was varied from 20 to 45 kHz where

Q-switched pulses are observed. The pulses were

obtained with a generated laser wavelength at 1560

nm with 22.6°C of Hi-Bi fiber loop (room

temperature). The results shown typical actively Q-

switched pulses behaviour, with the increase of the

repetition rate the pulse increases the build-up delay

time and pulse widens as pulse amplitude decrease.

Figure 6: Actively Q-switched pulses profile on repetition

rate variations for generated laser wavelength at room

temperature.

Pulse duration and pulse energy for different

repetition rates are presented in Figure 7.

Measurements were performed with the same

actively Q-switched laser operation characteristics

shown in Figure 6.

Figure 7: Pulse duration and pulse energy on repetition

rate variation for generated actively Q-switched laser

wavelength at room temperature.

The estimated pulse energy and the measured

pulse duration are presented in a repetition rate from

20 to 45 kHz. The maximal pulse duration of 1.57 μs

is obtained at the maximal repetition rate of 45 kHz

where the pulse maximal widening is observed. The

maximal pulse energy of 269 nJ is obtained with the

minimal repetition rate of 20 kHz. With the

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repetition rate increase, pulse duration increase and

pulse energy decrease, as it is observed. The

obtained results show a typical performance of

actively Q-switched pulses generation.

Figure 8 shows the measured pulse durations on

repetition rate variations for actively Q-switched

pulses obtained in different generated laser

wavelength tunings. Measurements were performed

in a repetition rate from 20 to 50 kHz. A group of

ten pulse durations was obtained for laser

wavelengths tuned from 1542 to 1560 nm each 2

nm. Pulse duration increase with the repetition rate

increasing is observed. As it is shown, the pulse

widens increase for longer laser generated

wavelength tuned. As a result, the maximal pulse

duration of 1.67 μs is obtained at maximal repetition

rate of 50 kHz for the laser emission tuned to 1560

nm. The maximal pulse duration obtained with a

repetition rate of 50 kHz varies with the laser

wavelength tuned in a range from 1.31 to 1.67 μs,

however, for the minimal repetition rate of 20 kHz,

the pulse duration varies in a narrower range from

482 to 648 ns. Although an increase of pulse

duration with longer laser wavelength tuned is

observed, the behaviour tendency in not clearly

defined.

Estimated pulse energy in function of repetition

rate variations for different laser wavelengths tuned

on actively Q-switched laser operation is shown in

Figure 9. The pulse energy was estimated for the

same pulse measurements obtained in Figure 8. The

results show a pulse energy decrease with the

increase of the repetition rate, a typical actively Q-

switched pulses performance.

Figure 8: Pulse duration on repetition rate variations of

different wavelength tunings for actively Q-switched laser

operation.

Figure 9: Pulse energy on repetition rate variations of

different wavelength tunings for actively Q-switched laser

operation.

The maximal pulse energy of 290 nJ is obtained

with the minimal repetition rate in which stable Q-

switched pulses are observed of 20 kHz and with the

generated laser wavelength emission at 1558 nm.

The maximal pulse energy (with repetition rate of 20

kHz) on different laser wavelength tunings varies in

a range from 231 to 290 nJ. In agreement with the

results obtained for pulse duration shown in Figure

8, a tendency of increase in pulse energy is also

observed for longer tuned wavelengths; however, the

increase is not clearly noticed between each

wavelength tuning. The actively Q-switched laser

performance of increasing of pulse duration and

pulse energy with longer wavelengths tuned is not

consistent at each variation of the repetition rate;

however, it is noticed as a global tendency observed

from figures 8 and 9 experimental and estimated

results.

4 CONCLUSIONS

In this manuscript, it is experimentally demonstrated

the use of a Hi-Bi FOLM as reliable, effective and

simple device to achieve a method for generated

laser wavelength tuning for ring cavity fiber laser

experimental setups. The results were obtained for

laser CW regime and for pulsed regime by the active

Q-switched technique.

The tuning range of the laser wavelength

depends on the Hi-Bi FOLM wavelength

transmission period. With the proposed Hi-Bi

FOLM, the tuning range is 26.72 nm in which single

wavelength emission is generated. In the tuning

limits, incipient dual-wavelength laser generation is

Hi-Bi Sagnac Interferometer Application for Wavelenght Tuning in CW and Actively Q-switched Erbium Fiber Laser

305

observed with separation between generated laser

lines corresponding to the Hi-Bi FOLM period of

30.32 nm.

The preferred wavelength transmission period of

the Hi-Bi FOLM transmission spectrum in which the

laser wavelength is generated depends on the losses

inside the laser cavity, which, together with the EDF

amplification spectrum modify the Hi-Bi FOLM

transmission spectrum to generate ideal conditions

where laser wavelength is emitted. This wavelength

is tuned by displacement of the Hi-Bi FOLM

transmission spectrum through temperature changes

application on the FOLM Hi-Bi fiber loop.

Laser wavelength tuning on Hi-Bi FOLM fiber

loop temperature variations slope of -1.47 nm/1ºC in

CW operation and -1.5 nm/1ºC in active Q-switched

operation is observed. For pulsed operation by the

active Q-switched technique, changes are observed

in the pulse characteristics depending on the

generated laser wavelength. Pulse duration and pulse

energy increase when the laser emission is obtained

at longer tuned wavelengths.

ACKNOWLEDGEMENTS

We gratefully thanks to Cátedras CONACyT,

CONACyT postdoctoral fellow 160248 and

CONACyT project grants 237855 and 255284.

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