Tunable fiber laser using fiber Bragg gratings integrated carbon fiber composite with large tuning range

Optics & Laser Technology 64 (2014) 302–307

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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Tunable fiber laser using fiber Bragg gratings integrated carbon fiber composite with large tuning range Shien-Kuei Liaw a,b,n, Chow-Shing Shin b, Wen-Fang Wu b,c a

Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan c Institute of Industrial Engineering, National Taiwan University, Taipei 10617, Taiwan b

art ic l e i nf o

a b s t r a c t

Article history: Received 2 March 2014 Received in revised form 12 May 2014 Accepted 23 May 2014

A wide-tuning range linear cavity C þL band tunable fiber laser is proposed in this paper. By using 3-point bending device to facilitate wavelength tuning of fiber Bragg gratings (FBGs), a scheme with two parallel strain-tunable FBGs (TFBGs) is demonstrated in L band operation. A large tuning range of over 22.5 nm with 0.1 nm precise resolution for each TFBG is obtained. The overlapping tuning range for two TFBGs is from 1564 to 1600.5 nm with 2 dB power variation. Using 10 M Erbium-doped fiber (EDF) and 100 mW pumping power, the stable lasing output power is measured at 1582.0 nm with threshold pump power and side mode suppression ratio (SMSR) of 10 mW and 50 dB, respectively. And the measured slope efficiency is 11.5% corresponding to quantum efficiency of 12.3%. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Linear cavity Tunable fiber laser Fiber Bragg grating

1. Introduction The first tunable laser was a dye laser invented in 1966 [1]. A tunable laser can be adjusted to produce a variable wavelength or frequency. This adjustable function allows the laser to be used for optical switching, network protection, equipment testing, medical treatment and wavelength routing [2–4]. In a wavelength division multiplexing (WDM) system the tunable laser is recognized as a backup light source for fixed wavelength laser replacement, with the motivating factors being cost savings and potentially higher system reliability. Tunable wavelength capability may significantly reduce costs for service providers because it eliminates the requirements for redundant equipment. Tuning the laser output across this range can be achieved by placing wavelength-selective optical elements (such as an etalon) into the laser's optical cavity to provide a particular wavelength selection. Temperature variation may cause the shift in the peak reflective wavelength of a distributed feedback (DFB) semiconductor laser. The tuning range for such a laser is however only a few nanometers. Multiple-prism grating arrangements, in several configurations, are used in diode, dye, gas, and other tunable lasers [5]. Other technologies used to achieve tuning ability include using multiple-prism grating arrangements for wide-

n Corresponding author at: Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan. E-mail address: [email protected] (S.-K. Liaw).

http://dx.doi.org/10.1016/j.optlastec.2014.05.020 0030-3992/& 2014 Elsevier Ltd. All rights reserved.

range tenability [6], discretely tunable lasers based on filtered feedback [7], tunable fiber ring lasers based on Stimulated Rayleigh Scattering in a non-uniform fiber [8], tunable external cavity lasers using an Asymmetric ring resonator reflector [9], tunable Brillouin fiber lasers based on a Fourier-Domain mode-locking source [10] and so on. Among various tunable lasers, tunable fiber lasers present the advantages of low intensity noise, thermal stability, excellent coupling into a single mode fiber and better compatibility with fiber components. In order to utilize WDM, Lband (about 1570–1605 nm) could be integrated with C-band (1530–1565 nm) to increase the capacity. Previous works proposed an L band tunable fiber laser design and/or characteristic such as Brillouin–erbium fiber laser (BEFL) with double Brillouin frequency spacing mechanism with -10 dBm for each signal [11], a multiwavelength Brillouin/erbium fiber laser with 40 nm tuning range in double-pass pre-amplification for the pump laser diode [12], using a high-nonlinear photonic crystal fiber to obtain 0.15 nm resolution and 45 dB side mode suppression ratio (SMSR) [13] and so on. Most of them are ring cavity based fiber lasers. In nature, the length of a fiber ring cavity laser is usually longer than linear cavity laser. As a result, the mode spacing is closer. In contrast, a shorter laser cavity may reduce the output power fluctuation due to less relaxation oscillation. This is the case for bulk or linear cavity fiber lasers. This paper investigates largetuning-range fiber Bragg gratings (FBGs) in a linear-cavity L band erbium-doped fiber laser. Laser characteristics such as gain fiber length, threshold pumping power, pumping slope efficiency and SMSR are measured and studied.

S.-K. Liaw et al. / Optics & Laser Technology 64 (2014) 302–307

2. Proposed tunable fiber laser scheme 2.1. Experimental setup The measured amplified spontaneous emission (ASE) spectra versus Erbium doped fiber (EDF) length across the C þ L band is shown in Fig. 1. In this measurement, an EDF of low Er3 þ ion concentration with signal absorption of 4.0 and 6.5 dB/m at 1480 and 1531 nm is employed. A standard forward pump scheme with an erbium-doped fiber amplifier (EDFA) in one-stage EDF construction is adopted. The pumping power is 270 mW at 1480 nm. We found that the C band ASE is established when the EDF is 7 M. The ASE spectrum also shifts to the longer wavelength region if the EDF length increases. As the EDF reaches 70 M in length only the L band ASE exists. If the laser cavity is too long, it has output power fluctuations and polarization mode competition. Using higher Er3 þ doping EDF may help to shorten the optimum EDF length and increase the laser output power more than the lower Er3 þ doping EDF does. It may also help to reduce the threshold pump power while the SMSR is slightly degraded.

303

TFBG is a valuable option for applications that require flexibility in the central wavelength. By compressing or straining (under tension) an FBG, it shifts towards shorter wavelength or longer wavelength, respectively. Using such an FBG in the tunable Erbium-doped fiber laser (TEDFL) body, the lasing output power of a tunable fiber laser is given using the following equation [14]. P Las ¼ ð1  R2 Þð1  ε2 ÞP out R

ð1Þ

In practical lasers, R2 is realized by an FBG in this work. ε2 is the intra-cavity loss for the linear cavity; Pout R is the laser power output measured at the right hand side of tunable FBG. TFBGs with appropriate original wavelengths are selected for the individual bands to obtain a Cþ L band laser. As shown in Fig. 2 (a), the rear cavity end could be a broadband fiber mirror (BFM), a 2  2 fiber loop mirror (FLM) or a 3-port optical circulator (OC). The available BFM (Model No. FM-2C, WT& T Inc.) in lab has a reflectivity of 98% for 1500–1600 nm and 80% for 1480 nm pump laser, respectively. It is a compact component with high price. The lasing power is a little bit unstable due to mutual injection effect. The 2  2 FLM is the cheapest among all components but suffers splitting loss. For example, the fiber loop mirror with 30/70 split ratio is designed to be 16% output coupler based on the following equation [15]. T ¼ ð1  γ Þ2 e  2αL ð1  2kÞ2

Fig. 1. The measured ASE spectra versus EDF lengths across the C þL band.

ð2Þ

where T is the fiber loop reflector transmission, γ is the excess loss, α is the fiber propagation loss, k is the split ratio and L is the fiber length. The loop-back 3-port OC inside the linear cavity laser plays as a quasi-ring cavity in which the lasing signal travels unidirectional on that loop, which may help to partially reduce the mutual injection noise for obtaining the stable lasing power. Therefore, the loop-back 3-port OC will be experimentally investigated in this paper. Fig. 2 (b) shows the proposed wide band TEDFL scheme using either component (1), (2) or (3) indicated in Fig. 2(a) as the rear cavity end. There are two 1  2 OSW pairs on the right hand side. The TFBGs in between for generating the C band and L band

BFM

FLM 1

2

3-port OC 3

1x2 TFBG1 1x2 OSW OSW

1x2 OSW C band

(1),(2) or (3)

C+L WDM

EDF1

SMF

TFBG2 VOA 1x2 1x2 OSW TFBG3 OSW

ISO Pump LD

EDF2

C+L WDM

OSA

PM

TFBG4 L band

Fig. 2. (a) The possible rear cavity end candidates of BFM, FLM or OC, and (b) the schematic diagram of the proposed Cþ L band TEDFL, the total tuning range is doubled by inserting 1  2 OSW pair in parallel with TFBGs inside. BFM: broadband fiber mirror; FLM: fiber loop mirror; OC: optical circulator; EDF: erbium-doped fiber; TFBG: tunable fiber Bragg grating, OSA: optical spectrum analyzer, PM: power meter, ISO: optical isolator.

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signals are divided into two groups, assigned in parallel to either of the 1  2 OSW pairs. By compressing or straining those TFBGs, their overlapping spectra could cover the whole C þ L band. Inside the 1  2 OSW are two parallel TFBGs (i.e., TFBG1 and TFBG2) in the upper path of 1  2 OSW pair for the C band branch, and another two parallel TFBGs' (i.e., TFBG3 and TFBG4) in the bottom path for the L band branch. The lasing wavelength either in the C band or L band depends on whether the upper path or the bottom path of the 1  2 OSW pair is chosen. The remaining fiber components inside the Cþ L band TEDFL form the cavity of this tunable laser. These include a rear cavity end, C þL band WDM coupler and a segment of EDF1, another C þL WDM coupler and a VOA. Whether the C band signal or L band signal will lase depends on the switching status of the 1  2 OSW pair. The gain competition between the C band and L band signal is negligible because only one signal in either band is allowed for lasing at one time. For the C band cavity EDF1 is the sole gain medium for the C band laser. For the L band cavity the total EDF length is the summation of EDF1 and EDF2. Since the L band needs more pumping power than the C band, EDF2 is used for transferring the C band ASE into the L band ASE as well as reusing the residual pumping power. In general EDF2 is longer than EDF1. The pumping power is in forward pumping scheme with lower noise. Nevertheless, backward pumping scheme could be used for residual pumping power for reuse purpose. Here, the original reflected wavelengths of TFBG1 and TFBG2 could be selected as, for example, 1532 and 1553 nm for C band; and the original reflected wavelengths of TFBG3 and TFGB4 could be selected as, for example, 1573 and 1586 nm for L band. Thus, the whole C þL band could be covered using only four TFBGs. The VOA at the right hand side of OSW is used for sweeping wavelength equalization in the entire Cþ L band. To overcome the extra insertion losses induced by OSWs, VOA and so forth, a pump source with pumping power equal to or larger than 100 mW should be considered. 2.2. Tunable FBG design FBG is a valuable option for an application that requires flexibility in the central wavelength. By compressing or straining

(under tension) an FBG, it shifts towards shorter wavelength or longer wavelength. Although FBG wavelength temperature tuning is an alternative choice that offers a compact and sometimes cheap solution, the main drawback is its tuning speed. This is especially a concern when cooling down is needed to obtain a shorter wavelength. In this case the temperature and pressure are fixed while the strain is varied. From the solid mechanics theory [16] the transverse displacement (ν) is related to the longitudinal strain (ε) applied to the FBG for tuning purposes. My I

ð3Þ

PL3 48EI

ð4Þ

Eε ¼ and v¼

where E is the elastic modulus, I is the moment of inertia, M (¼PL/2) is the bending moment, P is the load, the L here is the span of the 3-point bending supports and y is the distance from neutral axis to that point where the bending strain is applied. By combining Eq. (3) and Eq. (4) and letting M (¼PL/2) we obtain the relationship between ν and ε as

ε¼

48vy

ð5Þ

 2L2

The wavelength shift is in linear proportion to strain [17] as:

ΔλB ¼ λB ðK ε ε þ K T ΔTÞ

ð6Þ

Here, λB is the peak wavelength of an FBG, K ε is the strain constant and K T is the thermal constant. If the temperature remains constant, the tuning wavelengthðΔλB Þ is:

ΔλB ¼ λB

48K ε νy  2L2

ð7Þ

From Eq. (7), the tuning wavelength is proportional to the transverse displacement (ν). To investigate a TFBG, we embedded an FBG in the outer lamina of a four-lamina carbon fiber composite as shown in Fig. 3 (a). The composite with the embedded TFBG was attached to a 3-point tuning device with instant adhesive glue. By applying either tension or compression with a precision

Fig. 3. (a) Schematic diagram of the 3-point bending device for tuning a fiber Bragg grating, (b) the un-deformed element, and (c) the deformed elements.

S.-K. Liaw et al. / Optics & Laser Technology 64 (2014) 302–307

ΔL

0:01 ¼ 100 με ε¼ ¼ L 100

20

Peak Power ( mW )

screw device, we can apply transverse displacement in either direction to easily obtain a tunable range of 710 nm. The dimensions and parameters of the 3-point tuning device are summarized as ν ¼ 70.01 mm, y¼ 0.6625 mm, L ¼10 cm, and the original wavelength λB is at 1544.3 nm. The thickness of the single layer lamina composite is 0.6 mm. Fig. 3 (b) shows the unformed element from the neutral axis where the FBG within the composite beam is free of strain. In Fig. 3 (c), when the transverse displacement (ν) is negative, the center of curvature of the deformed element is below the neutral axis, the top of the element is under tensile strain, and the bottom of the element (the laminate with embedded FBG) is under compressional strain. This observation indicates that the strain of FBG within the composite beam depends on parameter y which, in turn, reflects whether the position of the center of curvature is above or below the neutral axis. Thus, the FBG wavelength varies and provides the required optical characteristics throughout a required tuning range. The 3-port bending device is 10 cm (L) in length, the length of embedded FBG is 2 cm, 3-dB BW of FBG remains within 0.2 nm in the whole tuning range. To find the corresponding wavelength drift under the condition that ΔL ¼0.01 mm, ε is obtained as

5M 10M 15M

15

10

5

0

0

10

20

30

40

50

60

70

80

90 100

R(%) of FBG at 1582 nm Fig. 4. The simulated result shows the output power versus TFBG reflectivity for a linearity-cavity fiber laser at different EDF lengths. The wavelength is at 1582 nm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

ð8Þ

OC 1

Assuming kε ¼ 0:645 as a standard value and Δλ ¼ λ U K ε U ε, the step of wavelength shifted will be

Δλ ¼ ð1550 nmÞð0:645Þð100 μεÞ ¼ 0:10 nm

305

C+L 2 WDM 3 ISO

ð9Þ

EDF

1x2 1x2 OSW TFBG5 OSW VOA

OSA

TFBG6 PM

Pump LD

3. Simulation/measurement and results

Fig. 5. The experimental setup for L-band TEDFL. The pumping power is in the forward pump direction and a 3-port OC is used for signal loop back.

In this section, we present the simulated and/or measured results for several laser parameters such as EDF length, FBG reflectivity, and pump power. A few realistic parameters were taken into consideration during the calculation. The pump power, the central wavelength and 3-dB FBG bandwidth are 100 mW, 1582.0 nm and 0.2 nm, respectively. The EDF has a core diameter of 6 μm and Er3 þ concentration of 800 ppm, and the numerical aperture and meta-stable lifetime are 0.22 and 10 ms, respectively. The power absorptions of EDF are 12.0 and 19.2 dB/m at 1480 and 1531 nm, in sequence. The gain fiber was core-pumped with a wavelength stabilized 100-mW and 1480-nm laser diode through a wavelength division multiplexing (WDM) coupler. Next, we set up the condition for lasing threshold for fiber laser, which was reached when the optical gain of absorption is balanced by the summation of total losses experienced by light in one round trip of the laser cavity. By assuming steady-state operation, it can be shown that [5] R1 R2 expð2g threshold lÞexpð 2αlÞ ¼ 1

ð10Þ

where R1 is the TFBG reflectivity, R2 is the reflectivity (assume 90%) of the rear cavity end, l is the length of EDF, exp(2gthresholdl) is the round-trip threshold power gain, and expð 2αlÞ is the round trip power loss. The loss inside fiber cavity also includes signal absorption and scattering. We then varied the FBG reflectivity R1 from 0% to 100% step by step in 5 M EDF to obtain the bold (red) curve in Fig. 4. By changing the EDF lengths to 10 M and 15 M, respectively, and following the same procedures, we obtained the other two curves in Fig. 5. In all three cases, FBG with very low reflectivity may cause large cavity loss; while the high reflectivity may generate smaller laser output. The FBG at 1582 nm and around 13% reflectivity could be the optimal option for the maximum power lasing. We find that 5 M EDF has a flatter spectrum compared to 10 and 15 M EDF cases. The 10 and 15 M EDFs have similar spectra meaning that the cavity gain and loss is balanced for the extra 5 M EDF.

Without loss of generality, an experimental demonstration of L band fiber laser scheme is shown in Fig. 5 where the pumping power is in the forward pump direction and an OC is used for signal loop back. We investigate only the L band fiber laser for feasible study as it is more complicated than that of the C band fiber laser. Either TFBG5 and TFBG6 is tuned by compression to shorten the wavelength or by strain to lengthen the wavelength. Three EDF lengths of 5, 10 and 15 M were used to observe the energy conversion from pump to lasing signal. The high Er3 þ concentration may shorten the cavity length as well as reduce the threshold pump power. Using 1582.0 nm fiber laser and 100 mW pump power at 1480 nm, Fig. 6(a) shows the output power versus pump power in three EDF lengths. The 5 M EDF has the lowest threshold pump power at 9 mW yielding a lowest lasing power of 7.8 mW at 100 mW. The threshold power is 15 mW for 15 M EDF. An output power of 11.2 mW was achieved for a launched power of 100 mW. In contrast, 15 M EDF has the highest laser power and pumping slope amongst the three different EDF lengths. The L–I curves are quite linear meaning that the absorption is not yet saturated. Fig. 6 (b) shows the transfer efficiency versus pump power at 1582.0 nm under different EDF lengths. The transfer efficiency here is defined as

η¼

P Las th ðP in p  pp Þ

ð11Þ

where η is the laser transfer efficiency, Pin R is the input pump power and Pth p is the threshold power. We found that the transfer efficiency increases as the EDF length increases with values around 8%, 11.5% and 13.5% for 5 M, 10 M and 15 M EDF, respectively. The results indicate that 5 M EDF is not long enough to convert 1480 nm pump power into L band ASE during the population inversion process.

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EDF= 5M EDF= 10M EDF= 15M

14

Peak Power (mW)

12 10 8 6 4 2 0

0

10

20

30

40

50

60

70

80

90

100

90

100

Pump (mW) 25

EDF= 5M EDF= 10M EDF= 15M

Efficiency %

20 15 10 5 0

0

10

20

30

40

50

60

70

80

Pump (mW) Fig. 6. The experimental results for (a) lasing output power against pump power, and (b) transfer efficiency against pump power for different EDF lengths in a forward pump scheme. All data are measured at 1582 nm.

For wavelength tuning experiments, Fig. 7 (a) shows that the power variance is 2 dB in 22.5 nm tuning range, from 1564 to 1586.5 nm, with the original TFBG5 centered at 1573 nm. Fig. 7 (b) shows that the power variation is 1 dB in 22.5 nm tuning range, from 1578 to 1600.5 nm, with original TFBG6 centered at 1586 nm. The tuning range is about 50% larger than our prior work of 15 nm [14]. Nevertheless, the laser power level is not degraded too much even TFBG5 at its boundary wavelengths of 1564 and 1586.5 nm, and TFBG6 at its boundary wavelengths of 1578 and 1600.5 nm, respectively. The measured SMSR ranges 46–50 dB at pump power of 100 mW. No polarization mode competition is observed. Note that 8.5 nm overlapping region, from 1578 to 1586.5 nm, could be obtained either by straining TFBG5 or compressing TFBG6. Thus, continuous and precise tuning with 0.1 nm resolution per step is possible using this technology. As the pump power increases, the optimum EDF lengths and lasing power will also increase accordingly. In principle, the multiwavelength laser output can be realized in EDF fiber lasers. However, the challenge arises from the gain competition among channels as well as the mutual injection effect due to homogeneous broaden characteristic of EDF. To conquer these problems, gain medium (i.e., EDF) for certain wavelength determined by the corresponding TFBG should be separated. In other words, N number of lasing cavities are required for simultaneous N lasing wavelengths. An example may refer to our previous study by using a parallel pump-shared linear-cavity scheme to obtain a laser array [18] where the signals are of fixed wavelengths. The minimum channel spacing is 0.1 nm in this work which indicates that we could obtain up to 365 channels (from 1564 to 1600.5 nm). The signals power can be boosted up by a master oscillator power amplifier (MOPA).

4. Conclusions A versatile laser source should allow the user to choose the necessary wavelength and/or to scan over a certain wavelength span. A C þL band tunable fiber laser is proposed and large range tunable fiber laser in L band is demonstrated. The TFBG could be used to tune the desired wavelength precisely and quickly. Using 1  2 OSWs and TFBGs, the tunable wavelength range could cover the L band from 1564 to 1600.5 nm with less than 2 dB power variation. Both TFBGs have tuning range of 22.5 nm with precise tuning resolution of 0.1 nm. A measured stable lasing output power at 1582.0 nm have threshold pump power and SMSR of 10 mW and 50 dB, respectively, when 10 M EDF and 100 mW pumping power are used. The measured slope efficiency is 11.5% corresponding to a quantum efficiency of 12.3%. This high performance and cost-effective tunable fiber laser has potential applications in optical measurement, network protection, back up light source and optical communication.

Acknowledgments The work was supported in part by NSC of Taiwan under Grant number NSC 102-2221-E-011-008. The authors thank Mr. C. L. Lai and Mr. C.C. Fan for their help in preparing the paper. References

Fig. 7. Superposed output spectra of the BFM-based TEDFL (a) with 2 dB power variation from 1564 to 1586 nm, and (b) with 1 dB power variation from 1578 to 1600.5 nm, respectively.

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