1. Introduction

Raman gain available virtually at any wavelength within transparency of optical fiber is a main attraction of this type of gain media compared with glasses doped with rare-earth ions and bismuth. Central wavelength of the Raman gain depends on the pump source and its bandwidth could support pulses in femtosecond regime. There is a critical aspect in high-power Raman lasers and amplifiers – they are essentially core-pumped devices, since double-clad pumping scheme offers poor efficiency. Therefore, a relatively high pump power launched into a single-mode fiber core is required to achieve noticeable gain [1,2]. Commercial laser diodes, however, are capable to produce only sub-Watt powers in single-mode fiber [3]. For this reason Raman amplifiers frequently use complicated pumping scheme: high-power cladding-pumped fiber laser followed by the Raman convertor/laser shifting the pump wavelength to required spectral range [4]. The promising pumping scheme for Raman fiber amplifiers could utilize high-power semiconductor disk laser (SDL) with diffraction-limited output beam, instead of using conventional in-plane diode lasers. Another advantage of SDL essential for pumping Raman amplifiers is that they could produce low-noise radiation [5,6].

Nowadays, due to lack of efficient low-noise sources, counter-propagating pumping scheme for Raman amplifiers is superior over co-pumping scheme and is predominant. Co-pumping, however, would significantly improve signal-to-noise ratio on condition that pumping source is low-noise. It has been shown recently that the relative intensity noise (RIN) of semiconductor lasers can reach extremely low levels, provided that the laser operates in the so-called class-A regime [7,8]. Shot noise limited operation has been demonstrated with semiconductor disk laser, which typically employs high-Q cavity [7]. Emerging of low-noise high-power disk lasers operating in a wavelength range 1.3-1.6 µm could radically change the technology of Raman fiber amplifiers.

Mode-locked Raman fiber lasers demonstrated to date use Raman fiber wavelength convertors as a pumping source [9,10]. In this paper we demonstrate fiber Raman laser mode-locked by nonlinear polarization evolution and pumped by 1.3 μm disk laser producing stable pulse train with pedestal-free 1.97 ps pulses at 1.38 µm in normal dispersion regime.

2. Design and performance of 1.3 µm semiconductor disk laser

The so-called wafer fusion technology used for combining disparate materials in various optoelectronic devices allows integrating semiconductor materials with different lattice. This technique allows the integration of disparate semiconductor materials, e.g. GaAs and InP, which cannot be grown monolithically. Recently, this technique has been applied for the first time to a high-power InP based 1.3 µm and 1.57 µm disk laser revealing a good performance [5,6]. Similar wafer-fused optically-pumped 1.3 µm disk laser is used in this study for pumping Raman fiber lasers. The active medium of SDL was grown by LP MOVPE on InP substrate. The periodic gain structure comprises 10 compressively strained (1%) AlGaInAs quantum wells. DBR grown by solid-source MBE consists of 35 pairs of quarter-wave thick Al_0..9Ga_0.1As and GaAs layers. The wafers have then been processed using a 2-inch wafer fusion technique, as described in [11]. After the fusion step, the InP-substrate was selectively etched by wet etching and cut into 2.5 × 2.5 mm² chips, which were bonded with GaAs substrate down using AuSn solder to a copper plate. Since the gain mirror operates under intense pumping conditions, thermal management is a crucial issue in the disk laser. In our laser the generated heat was conducted to a heat sink using a transparent intracavity heat spreader. A 3 × 3 × 0.3 mm³ intracavity natural type IIa diamond heat spreader was bonded on the top surface of the sample with de-ionized water. InP cap layer and surface of diamond are pulled together by intermolecular forces of water. The sample is further mounted between two copper plates with indium foil in between to ensure reliable contact. The topmost metal plate had a circular aperture for signal and pump beams, while the bottom copper block was cooled by water. The cavity of the disk laser was of V-type and composed of a 97.5%-reflective plane output coupler, curved mirror and the gain mirror. The gain mirror was pumped with 980 nm fiber coupled diode laser. The pump is focused onto the gain mirror to a spot of 180 μm in diameter. The cavity was simulated numerically to ensure that the mode size at the gain mirror matches the pump spot. 2 W at 1.3 μm from disk laser has been obtained in a single-mode fiber owing to good beam quality factor M²<1.5.

3. Mode-locked Raman fiber laser pumped by disk laser

The SDL was first tested as a pumping source of continuous-wave linear cavity Raman laser. The Raman fiber laser cavity comprised 650 m of highly nonlinear fiber with 25 mol% of GeO₂ in the core exhibiting the core/cladding index difference of ∆n = 0.03, numerical aperture of 0.25 and Raman gain of g₀ = 21 dB/(km × W). Mode field diameter and losses of the Raman fiber at 1.38 μm are 8.5 μm and 2.5 dB/km, respectively. It should be noted that high nonlinearity of the fiber used for Raman conversion allows for relatively short length of the cavity which is typical few kilometers long [9,10]. The linear cavity terminated by high-reflective dielectric mirror and 80% fiber reflector. The output characteristic and spectrum of cw Raman laser are shown in Figs. 1(a) and 1(b), respectively.

 

Fig. 1 (a) Output characteristic of continuous-wave Raman laser. Pump beam coupling efficiency from SDL to single-mode fiber is 70% at highest power of 1.8 W. Optical isolator placed between SDL and Raman laser induces the loss of 0.5 dB. (b) Optical spectrum of 1.38 µm continuous-wave Raman laser and spectral component of residual pump at 1.3 µm. Pump power is 1 W.

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Raman spectrum contains first Stokes component peaked at 1.38 µm and second Stokes. The residual pump is seen at 1.3 µm. Threshold pump power for Raman generation was 290 mW. 1.1 W of frequency converted power has been achieved at 1.8 W of launched pump power revealing a high efficiency of this pumping scheme of 65%. Linewidth of pump source at 1.8 W output power is measured to be 5 nm.

Figure 2 shows experimental setup of mode-locked Raman fiber laser. Ring cavity comprises polarization-dependent optical isolator which ensures unidirectional propagation and enforces the passive mode-locking through nonlinear polarization evolution. Zero dispersion wavelength is 1530 nm and dispersion at 1370 nm is −12.88 ps/(nm × km), therefore the laser operates in normal dispersion regime without any means for dispersion compensation. The Raman fiber is pumped through 1.3/1.38 µm fiber coupler and output signal is taken from 5% output coupler. The threshold of mode-locking regime was 340 mW of pump power. The pump power was limited to 1 W, since above this value an optical damage of cavity components specified for telecom applications could be occasionally observed. Though the mode-locked laser uses single-pass pumping scheme, the highly nonlinear Raman fiber, ensured high conversion efficiency. The measurements reveal that unconverted pump was always below 20%.

 

Fig. 2 Schematic of mode-locked Raman fiber laser.

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Output characteristic of modelocked laser is shown in Fig. 3 . An average output power up to 70 mW has been obtained for pulse operation at first Stokes wavelength.

 

Fig. 3 Output characteristic of modelocked laser versus launched pump power. Pump power is limited to 1W due to loss induced by an optical isolator.

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The plot in Fig. 4(a) shows survey Raman spectrum with first Stokes component at 1.38 µm, and residual pump. Figure 4(b) presents detailed spectrum of mode-locked pulse train with spectral width of 1.8 nm for pump power of 0.8 W.

 

Fig. 4 (a) Spectrum of mode-locked pulses centered at 1.38 µm and unconverted pump at 1.3 µm and (b) detailed spectrum of mode-locked pulses.

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The traces of Fig. 5 show the pulse trains under different pumping conditions exhibiting the envelope which bandwidth gradually increases with the pump power. It should be noted that the start-up process in mode-locked Raman laser is essentially different from the starting scenario exhibited by lasers with rare-earth fiber as a gain medium. Slow gain relaxation dynamics in rare-earth doped glasses (100 µs - 10 ms) allow for efficient energy storage in the laser cavity which usually provokes the evolution to steady-state mode-locking through the Q-switching instability [12]. On the contrary, in a Raman fiber laser exhibiting fast gain dynamics,${\tau }_{re\mathrm{cov}ery}^{Ramangain}\text{<<}{\tau }_{roundtrip}^{cavity}$ _, the tendency to Q-switching instability is strongly suppressed. Consequently, the mode-locked pulse train develops directly from continuous-wave noise radiation, while the number of pulses in a train increases with the pump power which determines the number of time slots filled with the pulses. Eventually, for sufficient pump power, the transition to actual continuous-wave mode-locking occurs when all the time slots are filled and uniform pulse pattern builds up without low-frequency envelope, as seen from Fig. 5.

 

Fig. 5 Mode-locked pulse train as observed from oscilloscope for different pump powers. Pulse repetition rate: 0.3 MHz.

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The autocorrelation of 1.97 ps pulses are shown in Fig. 6 . It should be noted that pedestal-free operation has been achieved without spectral filtering [9]. Owing to relatively short-length cavity and, consequently, low value of cavity dispersion of −7.41 ps/nm, the time-bandwidth product of the pulses is only 1.36 × higher than the transform limit. The output average power in mode-locked regime was 70 mW for pump power of 1 W.

 

Fig. 6 Autocorrelation of the pulse at 0.8 W of pump power with Gaussian fit showing pulse duration of 1.97 ps.

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It should be mentioned that recently we have demonstrated 1.6 µm Raman fiber laser pumped by disk laser and mode-locked by SESAM which operates in anomalous dispersion regime [13]. 2.7 ps pulses with time-bandwidth product of 0.69 have been achieved. On the contrary, the laser presented here has the cavity with normal cavity dispersion and, therefore, has superior potential for high energy pulse generation [14].

4. Conclusion

In conclusion, 1.38 µm mode-locked Raman fiber laser pumped by 1.3 µm wafer-fused semiconductor disk laser is demonstrated. Nonlinear polarization evolution combined with a highly-nonlinear fiber allows for relatively short-length cavity of ~650 m. Laser working in normal dispersion regime demonstrates stable operation without dispersion compensation with 1.97 ps pedestal-free pulses only 1.36 times over transform limit. Efficient low-noise pumping scheme offered by disk lasers demonstrates the promising potential for Raman fiber lasers and amplifiers.