1. Introduction

Gain switched laser diodes (GSLD) have attracted much interest for years, as they offer the opportunity to generate optical pulses with repetition rates up to several GHz with durations in the picosecond range by simply applying an electrical signal generator. Operation of GSLDs was first demonstrated at wavelengths of 1.3 µm and 1.55 nm [1–3] and recently at 1 µm [4]. By combining GSLDs with optical fibers and using linear and nonlinear pulse compression techniques, the possibility of dechirping pulses emitted by GSLDs down to the fs regime was demonstrated [3,5], as well as the power scalability from the mW to the several hundred Watt regimes [3–6].

Due to the large range of accessible pulse durations, and repetition rates, the simple setup as well as the power scalability, GSLDs are promising candidates for micromachining applications such as drilling and cutting. Furthermore, they can be used for frequency doubling and tripling, seeding OPAs, and for pump probe measurements [7–9].

Besides these applications of GSLDs it would be useful to assign the pulse characteristics and the flexible repetition rate to wavelength regions in which no GSLDs exist up to date. This would offer the opportunity to seed neodymium, bismuth and erbium doped fiber amplifiers with picosecond pulses of different GSLDs in the wavelength range between 1 and 1.6 µm. Moreover, it is desirable to have just one GSLD based source that provides a large variety of ps pulses with different wavelengths in the range between 1 and 1.6 µm to seed different gain media or to serve as pump source for pulsed Raman amplifiers [10]. Such sources could also be used as low cost robust broadband transmitters for spectrum sliced WDM systems offering the additional possibility of temporal gating in optical networks [11–13]. An additional application would be multispectral LIDAR [14].

A filtered supercontinuum source, as reported by Chen et al. [15], would have such properties as well, but the authors observed that some tens of kW peak power is needed to convert energy from 1060 nm to 1600 nm. Additionally, by using continuum generation in a photonic crystal fiber (PCF), the achieved conversion efficiency is low, while due to the anomalous dispersion of PCFs in the NIR wavelength range the effect of modulational instability causes temporal pulse splitting and noise [16]. Mussot et al. reported on a supercontinuum source based on a frequency-doubled passively Q-switched Nd:YAG microchip laser [17]. The generated continuum covered a broad range from the visible to the NIR, however, the necessary pulse energy was higher than 1 µJ and the repetition rate had a fixed value of 6.7 kHz. Lin and his associates published a work on a frequency doubled all-fiber MOPA generating multiple Raman Stokes wavelengths, but they used pulse durations between several tens and hundreds of nanoseconds [18]. Such long pulse durations do not come into consideration for most of the above mentioned applications. M. Liao et al. presented a five-order SRS and supercontinuum generation in tellurite microstructured fiber recently [19]. However microstructured fiber has disadvantages like high costs and splice losses. D. A. Grukh and his associates also presented a cascaded SRS source, but they used pulse energies in the µJ regime and pulse durations in the ns range [20]. Additionally, in their work the authors did not investigate the Stokes energy shift from one order to the next higher one and the Stokes pulse durations.

In our work we present an all-normal dispersion all-fiber continuum source based on a GSLD emitting pulses at 8 different central wavelengths ranging from 1.06 µm up to 1.59 µm and operating with a repetition rate in the MHz regime. The generation of new frequencies was accomplished by cascaded Raman scattering in a commercially available normal dispersive standard fiber. Seeding at a wavelength of 1064 nm, Stokes orders at central wavelengths of 1120, 1175, 1240, 1306, 1382, 1474 and 1570 nm were generated with pump pulse energies of just 81 nJ. The energy shift between the Stokes orders was investigated as well as the spectral and temporal characteristics of Stokes pulses after propagation in the Raman fiber.

2. Experimental setup

The setup of the reported GSLD based all-fiber multicolor laser system is shown in Fig. 1 and consisted of 3 stages: the seed source, a fiber amplifier and a fiber-based Raman converter. A narrow bandwidth GSLD with a central wavelength of 1064 nm (PicoQuant GmbH) was used as seed source. The repetition rate was tunable between 2.5 and 40 MHz. We operated the diode at a repetition rate of 2.5 MHz to have the highest available pulse energy for the nonlinear power conversion. The amplification of GSLDs in three ytterbium doped fiber amplifier stages was investigated in detail in Ref [6]. The optimized amplifier setup consisted of two core pumped fiber amplifiers, based on a single mode ytterbium doped fiber with a core diameter of 4 µm (YDF 1, YDF 2), followed by a third amplifier stage, based on a double clad ytterbium doped fiber (YDF 3) with a core diameter of 15 µm. While the first two fiber amplifiers were pumped by laser diodes emitting up to 600 mW, a laser diode with an output power of 2.5 W was used to pump the double clad fiber amplifier. All pump diodes had a central wavelength of 976 nm. The pump and seed signals were multiplexed with wavelength division multiplexers (WDM 1, WDM 2) for the core pumped amplifiers and with a multimode combiner for the double clad fiber. The amplifier section was followed by a 500 m long Raman fiber from OFS. It had an effective core area of 17.3 µm2 and an attenuation of 0.43 dB/km at a wavelength of 1450 nm and a Raman gain coefficient of 2.51 (W x km)-1. The dispersion was measured to be −4.1 and −0.3 ps/(nm x km) for wavelengths of 1510 and 1620 nm, respectively. The Raman fiber provided normal dispersion in the wavelength range between 1 µm and 1.6 µm, which was necessary to avoid modulation instabilities. In the Raman fiber, frequency conversion starting from 1064 nm up to 1590 nm occurred. The GSLD and the fiber amplifier stages were protected against backward reflections by fiber based Faraday isolators (ISO), which were designed for a central wavelength of 1064 nm. At the angle cleaved Raman fiber output the spectral and temporal characteristics of the Stokes pulses were measured.

 

Fig. 1 Setup of the GSLD based all-fiber Raman shifter system.

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3. Experimental results

At a repetition rate of 2.5 MHz the maximum pulse energy emitted by the GSLD was 160 pJ. For the measurement of the temporal pulse shape, an ultrafast photo diode (New Focus, Model 1014) with a bandwidth of 45 GHz and a sampling oscilloscope with a bandwidth of 70 GHz (LeCroy), were used. By using this specific measurement setup, it was possible to resolve pulse durations below 30 ps. When the GSLD was operated at maximum power, a pulse tail was observed in the temporal domain. According to the manufacturer, this pulse tail had its origin in multiple relaxation oscillations of the gain switched laser diode. While the main peak had a width of 74 ps at an amplitude level of 60%, a width of 1.05 ns at an amplitude level of 20% was measured. The saturated operation at maximum pump power of the amplifier stages resulted in an average output power of 0.72 W, corresponding to pulse energies of 288 nJ and an amplification factor of 1800. No indications for nonlinear pulse broadening of the seed pulses in the spectral and temporal domain were observed after the amplification, compared to the output pulses of the GSLD. A very detailed description of fiber amplifiers designed for the amplification of GSLDs at low repetition rates is given by our previous work in Ref. 6 and was not in the focus of this work. The optical power spectrum on logarithmic scale and the temporal pulse shape after the last fiber amplifier stage are shown in Fig. 2 . The peak-to-peak ratio between signal and amplified spontaneous emission (ASE) was measured to be better than 28 dB. The spectral full width at half maximum (FWHM) of the output pulse was below 0.5 nm.

 

Fig. 2 Power spectrum after amplification in third fiber amplifier stage (left) and corresponding temporal pulse shape (right).

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The amplified GSLD pulses served as Raman pump pulses and were propagated in the 500 m long Raman fiber. Due to the mode-field mismatch between YDF 3 and the Raman fiber and the splice loss, the maximum coupling efficiency to the Raman fiber was just 28%. The power spectra measured at the output of the Raman fiber are shown on logarithmic scale in Fig. 3(a) at six different transmitted pulse energies. At transmitted pulse energy of 17 nJ, a Raman cascade consisting of three Stokes orders was observed with central wavelengths of the Stokes pulses at 1130, 1190 and 1255 nm. As the transmitted pulse energy increased to 27, 38, 50, 65 and 81 nJ, additional Stokes orders appeared at 1330, 1413, 1508 and 1570 nm. A dependence of the Stokes pulse central wavelengths and spectral shapes from the transmitted pulse energy was observed. However, at a transmitted pulse energy of 81 nJ the central wavelengths were 1120, 1175, 1240, 1306, 1382, 1474 and 1570 nm and thus approximated the expected values 1116, 1174, 1234, 1309, 1389, 1480 and 1582 nm. A possible explanation for this observation is the dependence of the exact Stokes pulse wavelength on the dispersion profile and the pump pulse shape, as reported by Vanholsbeeck et al. [21]. The averaged frequency spacing between the Stokes orders was 12.97 THz, whereas the averaged FWHM of each Stokes order was 2.69 THz. The measured frequency spacing is in good agreement with the well-known Raman gain maximum for fused silica of 13.19 THz from the literature [22].

 

Fig. 3 (a) Power spectra after propagation in the Raman fiber at five different transmitted pulse energies on logarithmic scale (b) Temporal power distribution, measured with an ultrafast photodiode without spectral filtering plotted on linear scale.

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The temporal power distribution at the corresponding pulse energies is plotted in Fig. 3(b) on linear scale. The pump and the Stokes pulses are labeled by P and numbers between 1 and 7. For this measurement, the trigger output of the GSLD driver was used to calibrate the delay axis. It was observed that the initial pulse shown in Fig. 2 was split to distinct sub-ns pulses. This observation indicated a temporal walk-off of the pump pulse and the Stokes orders due to the dispersion accumulated in the 500 m long Raman fiber. This walk-off results in a temporal arrangement of the Stokes orders according to their wavelength from the leading edge to the trailing edge of the pulses. Therefore, at a transmitted pulse energy of 17 nJ, the four distinct temporal peaks at delays of −3 ns, −2.5 ns, −1.4 ns and 0 ns can be assigned to the Stokes orders and the pump pulse at 1255 nm, 1190 nm, 1130 nm and 1064 nm, respectively. As the transmitted pulse energy was increased to 27, 38, 50, 65 and 81 nJ, additional pulses appeared in the temporal domain and energy was transferred to the leading edge. This was expected, as additional Stokes pulses appeared in the red part of the spectral domain, while simultaneously the power contained in lower Stokes orders decreased. The measured temporal pulse walk-offs in the order of one ns in Fig. 3(b) are in good agreement with the values expected from the dispersion data.

An energy shift from lower to higher Stokes orders is obvious from Fig. 3. To investigate this in detail, the relative contribution of each Stokes line up to the sixth order to the overall energy of the Raman cascade was derived from the measured power spectra by integrating the corresponding spectral interval. This analysis does not account for the wavelength dependent attenuation in the Raman fiber. The result is shown in Fig. 4 . As the seventh order at a wavelength of 1570 nm appeared just at the maximum pump energy, there were not enough measurement data to take it into account in Fig. 4. At a transmitted pulse energy of 17 nJ, the contribution of the first Stokes order (1130 nm) to the measured pulse energy was 12%, while the contributions of the second and the third Stokes orders (1189 nm; 1258 nm) were 68% and 18%, respectively. As the pump energy was increased to 27 nJ, the contribution of the first and the second Stokes order decreased to 3% and 19%, respectively. Simultaneously the third order contribution increased to 65%, while a fourth order appeared. The origin of this energy shift is the depletion of the Raman gain by the next higher Stokes order, which is provided by a lower one.

 

Fig. 4 The energy content of each Stokes order except at 1570 nm in the whole Raman cascade is depicted in percent for different pump pulse energies.

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The pulse duration of the pump and of the 7^th Stokes order pulse at transmitted pulse energies of 81 nJ were measured for comparison. For this, both pulses were filtered spectrally by applying bulk interference filters to the free-beam section. For filtering the pump pulse, a narrow line width filter (bandwidth 3 nm) with a central wavelength of 1064 nm and a Gaussian shape was applied. A spectral filter with a central wavelength of 1570 nm was not available for filtering the 7th Stokes order. For this reason a filter with a Gaussian shape, a central wavelength of 1560 nm and a FWHM of 12 nm was employed. After spectral filtering, the optical spectra and pulse shapes were measured with a spectrometer and with the ultra-fast photodiode. The results for the pump pulse are shown in Figs. 5(a) and (b) . After propagating in the 500 m long Raman stage, the temporal pump pulse shape changed as well as the pulse duration. The initial pulse shape (see Fig. 2) with a short picosecond peak in the front and a nanosecond tail evolved into a smooth shaped pulse due to efficient Raman conversion. Additionally, the complex interaction of effects like dispersion and pulse shortening due to Raman conversion are main contributions to the change of the pump pulse shape. The simulation of this complex interplay will be content of future work. The spectrum and temporal pulse shape of the 7^th Stokes order is depicted in Figs. 5(c) and (d) with a pulse energy of 0.2 nJ, a central wavelength of 1556 nm, a spectral FWHM of 14 nm; its temporal FWHM was measured to be 608 ps. The deviation between the central wavelength of the filtered and unfiltered 7th order Stokes pulse has its origin in the wavelength mismatch of the employed filter. However, this filter wavelength mismatch should not have significant effect on the measured 7th order Stokes pulse duration, if an incoherent Stokes pulse is assumed. The oscillations in Fig. 5(c) originated from the interference filter, while the modulation in Fig. 5(d) had its origin in the low signal intensity that was coupled to the photodiode.

 

Fig. 5 Pump pulse spectrum and temporal pulse shape at maximum pump energy after spectral filtering of pump pulse [(a) & (b)] and 7^th order Stokes pulse [(c) & (d)].

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We operated our cascaded Raman shifter at different repetition rates between 2.5 and 40 MHz. As the repetition rate increased at constant average power, the number of Stokes orders decreased due to the lower pump pulse energy. By scaling the average power of the amplifier and using a GSLD with higher repetition rates it should be possible to use the same approach to set up a cascaded Raman shifter with repetition rates up to several GHz repetition rate. In addition, scaling the pump pulse energy should increase the energy content of the depleted Stokes orders to the nJ regime.

4. Conclusion

We presented an all-normal dispersion multi-colored all-fiber laser system in the NIR range, based on a GSLD. The presented picosecond pulsed laser source combines the advantages of GSLDs and fiber lasers in an all-fiber system, while providing picosecond pulses at 7 different Stokes orders between 1 and 1.6 µm. Power is shifted very efficiently from the seed wavelength to the different stokes orders. At a transmitted pulse energy of just 81 nJ, a pulse energy of 0.2 nJ was obtained at a wavelength of 1556 nm. With such a device, fiber amplifiers, doped with ytterbium, neodymium, bismuth and erbium can be seeded at wavelengths of 1064, 1120, 1175, 1240, 1306, 1382, 1474 and 1570 nm simultaneously. The demonstrated picosecond multicolored light source opens up new possibilities in spectroscopy, broadband optical communication networks, and multispectral LIDAR. In optical networks, such sources could be used to setup low cost robust broadband transmitter for a spectrum sliced WDM system. Additionally, the temporal Stokes pulse separation in the order of ns offers the possibility of temporal gating and secures a low wavelength channel crosstalk.