1. Introduction

Frequency comb generation has attracted significant interest from researchers due to its potential applications in dense wavelength division multiplexing (DWDM) system [1], waveform synthesis [2], remote sensing [3], photonics-to-frequency mapping [4], material characterization [5], ranging [6] and spectroscopy [7]. In certain applications, four wave mixing (FWM) cascades is exploited to obtain wide bandwidth frequency comb. In [1], the FWM cascades was utilized in a 50 m long photonics crystal fiber (PCF) to increase the number of 50 GHz spacing DWDM channels to 37 channels with an average power of 5.4 mW per channel. In another configuration using 8 cm long PCF, a broad spectrum from 450 nm to 1400 nm was achieved in an experiment that used 73 fs pulses at 75 MHz repetition rate as the seed for FWM cascades.

In common configurations, mode-locked lasers (MLL) are used as seeds for frequency comb generation. As the comb frequency pitch is related to MLL cavity properties, it is quite difficult to generate an arbitrary comb frequency pitch [8]. Realizing this challenge, continuous wave (CW) lasers are therefore utilized as an alternative to seed the frequency comb generation, which allows for freely tuned arbitrary comb frequency pitch in contrast to MLL approach. Therefore, current efforts are now ongoing in generating FWM cascades with CW lasers as seeds for frequency comb generation. In order to obtain FWM cascades with frequency components spanning over wide bands, schemes which utilize short fiber [9], three-pump seeds [10] and dispersion property of fiber [8] were proposed and experimentally demonstrated. Besides the wide bandwidth, high power frequency components are also important. A scheme with optical feedback [11] was therefore reported recently in order to enhance the optical power of FWM cascades.

High power CW pumps are usually necessary for seeding the generation of frequency combs in parametric gain medium of highly nonlinear fiber (HNLF) or PCF. The high power pumps are mostly obtained by amplifying the seeds using erbium-doped fiber amplifier (EDFA). However, stimulated Brillouin scattering (SBS) has a narrow gain spectrum (< 100 MHz), which restricts the amount of power transmitted into the HNLF or PCF. One of the techniques used to suppress the backward SBS is modulating the phase of the pumps such that the pumps spectra are broadened beyond 1 GHz [9–11]. In practice, the phase modulation is driven either periodically at certain few radio frequencies or randomly using pseudorandom bit sequence at bit rates of 3-10 Gbits/s. However, such schemes come with their drawbacks as well. Firstly, the complexity is increased due to the inclusion of phase modulator and its drive signals, which ultimately raise the total cost of system. Secondly, the phase modulation has impacts on the scheme performance. It was theoretically and analytically demonstrated that the phase modulation can cause large variations in signal and idler powers as well as reduce their optical signal to noise ratio (OSNR) [12]. It was also experimentally demonstrated that it could distort mark level and cause system penalty [13]. A technique was therefore proposed recently for SBS suppression without the use of phase modulation for generation of FWM cascades [8]. A specific tension plan applied on a HNLF was able to increase its SBS threshold to 30 dBm. While this technique can avoid the use of phase modulation, it requires precise dispersion management which may increase the complexity of FWM cascades design. Moreover, the scheme needs two external laser sources as seeds for FWM cascades to operate. Therefore, a scheme without the use of external laser sources and phase modulation is desired for cost effective generation of multi-wavelength using the FWM cascades technique.

In this paper, we report and experimentally demonstrate for the first time to the best of our knowledge, a self-seeded, cost-effective technique to generate multiple wavelengths based on the FWM cascades. The proposed scheme does not require external tunable laser sources for frequency comb generation as a result of the use of dual wavelength erbium-doped fiber laser (EDFL) as intracavity pumps. In this scheme, we overcome the need for pump modulation to suppress the SBS by utilizing tunable bandpass filters (TBFs) with full width half maximum (FWHM) bandwidth of 0.23 nm. The use of the TBFs makes the linewidth of EDFL sufficiently broad for SBS suppression. The exclusion of external laser sources and pump modulations makes the proposed scheme more cost effective for FWM cascades generation.

2. Experiment and operating principle

The experimental setup for the self-seeded FWM cascades is shown in Fig. 1 . The scheme is self-seeded, meaning that tunable laser sources required as pumps for FWM cascades are replaced by an intracavity dual wavelength EDFL. A dual selective element, a circulator, a commercial IPG erbium-doped fiber amplifier (EDFA), a polarization controller (PC), a 500 m long highly nonlinear fiber (HNLF) and a 90/10 coupler form the ring cavity EDFL. The circulator does not only ensure unidirectional direction for the oscillating lights of dual wavelength laser but also connects the dual selective element with the ring cavity EDFL. The EDFA provides light amplification and its gain bandwidth is from 1542 nm to 1565 nm with output power up to 30 dBm. The PC, which is used to control the polarization of the light is adjusted such that the four wave mixing (FWM) efficiency within the HNLF is optimum. Two oscillating wavelengths of the laser are set by the dual selective element, which is composed of a 3 dB coupler, two variable optical couplers (VOAs), two TBFs, a 95% reflective optical mirror and a circulator that acts as a mirror by connecting its first port to its third port. The desired wavelengths are selected by adjusting the wavelength of TBFs that are tunable over the C-band region and have FWHM bandwidth of 0.23 nm. The HNLF in this configuration serves two purposes. Besides stabilizing the EDFL by mitigating mode competition in the EDF, it provides a nonlinear gain medium in which FWM cascades are generated. The HNLF has a nonlinear coefficient of 11.5 (Wkm)⁻¹ and a zero dispersion wavelength at 1556.5 nm. Its dispersion and dispersion slope at 1550 nm wavelength are −0.1 ps/(nm-km) and 0.015 ps/(nm²-km) respectively.

 

Fig. 1 Experimental setup of self-seeded FWM cascades.

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In our proposed scheme, the pumps for FWM cascades come from the dual wavelength EDFL. For the laser to operate efficiently, both VOAs are adjusted such that the two oscillating waves have almost the same power. This is achieved by monitoring the peak power via 50% port of the coupler on 0.05 nm resolution optical spectrum analyzer (OSA). This is done in order to alleviate gain mode competition of erbium-doped fiber (EDF). In the cavity, the oscillations of these dual wavelength lasers generate new waves through FWM process in the HNLF. The previously created waves then generate new waves in a process called cascaded or multiple FWM as the waves propagate through the HNLF. The phase or intensity modulation for SBS suppression is not needed in this configuration because the 0.23 nm FWHM bandwidth of TBFs made the linewidth of the dual wavelength EDFL wide enough to avoid the generation of SBS. In this case, the dual wavelength EDFL acts as pumps and its FWM cascades are then tapped out of the cavity via the 10% port of the 90/10 coupler and its spectrum is measured using the OSA with 0.015 nm resolution.

3. Result and discussion

Figure 2 shows the spectra of FWM cascades as the output power of EDFA increases from 8.33 dBm to 23.41 dBm. In order to observe the number of FWM cascades as the EDFA output power increases, we fixed the dual wavelength EDFL (pumps) at wavelengths of 1561.73 nm and 1564.91 nm. The wavelengths were selected by tuning the TBFs in the dual selective element. The FWM cascades are counted if its OSNR is more than 10 dB with OSA resolution of 0.015 nm.

 

Fig. 2 Output spectra of FWM cascades including the two pumps as EDFA output power increases for (a) two waves at 8.33 dBm (b) four waves at 11.95 dBm (c) six waves at 20.57 dBm and (d) eight waves at 23.41 dBm.

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Figure 2(a) displays a spectrum of EDFL that acts as pumps for the FWM cascades. It is considered two waves case since the OSNR of the generated FWM cascades is less than 10 dB at 8.33 dBm EDFA output power. In this situation, the FWM process in HNLF helps to stabilize the EDFL by mitigating mode competition in EDF through transference of power from the stronger wave to the weaker wave, leading to the generation of new waves along the process. It is interesting to note in Fig. 2(a) that the EDFL is free from any self-lasing cavity modes inside the cavity. The self-lasing cavity modes are self-oscillation EDFL cavity modes around the peak gain of EDF [14,15]. The absence of self-lasing cavity modes is important in achieving a stable dual wavelength EDFL. As the EDFA output power increases, the dual wavelength EDFL, which acts as two strong pump waves, initiates the FWM cascades. As a result, the number of FWM cascades having OSNR larger than 10 dB also increases as shown in Figs. 2(b)–2(d). The detailed study of the FWM cascades evolution from the pump waves is reported in [16]. Including the pumps, four waves, six waves and eight waves are generated at EDFA output power of 11.95 dBm, 20.57 dBm and 23.41 dBm respectively. The two pumps distinguish themselves by being the strongest waves in the spectra. It can also be observed that the noise pedestal is present in the spectra. It comes from the amplified spontaneous emission (ASE) of EDFA but it can be substantially reduced by spectral filtering.

The tunability of wavelength spacing of the FWM cascades is then investigated for six waves case at the EDFA output power of 20.57 dBm. In order to get the optimum FWM cascades, it is necessary to adjust the PC when the wavelength spacing is tuned by tuning the pump wavelengths. Figure 3 demonstrates the output spectra with different wavelength spacing. In this observation, one pump is fixed at 1564.33 nm and the other pump is varied away from 1564.33 nm to study the widest possible wavelength spacing. The EDFA has limited gain bandwidth from 1542 nm to 1565 nm, so the farthest wavelength of the varied pump is 1542 nm. As depicted in Figs. 3(a)-3(b), the minimum and maximum wavelength spacings are 1.03 nm and 11.31 nm when the varied pump wavelength is at 1563.3 nm and 1553.02 nm respectively. When the varied pump was tuned beyond the wavelength of 1553.02 nm, for example at 1542 nm as shown in Fig. 3(c), the number of waves is reduced to four. This reduction is due to the phase mismatch of waves in the region that is far away from the HNLF zero dispersion wavelength at 1556.5 nm. On the other hand, the minimum wavelength spacing is limited by FWHM bandwidth of TBFs, while the dispersion property of HNLF restricts the maximum wavelength spacing. The wavelength spacing tunability can be improved if we use smaller FWHM bandwidth of TBFs and engineer the dispersion property of HNLF. It is also interesting to note that the wavelength spacing in this proposed scheme is continuously tunable as a result of the continuous tunability of TBFs.

 

Fig. 3 Wavelength spacing tunability with a pump fixed at 1564.33 nm and the other pump is varied at EDFA output power 20.57 dBm (a) minimum wavelength spacing of 1.03 nm for six waves (b) maximum wavelength spacing of 11.31 nm for six waves and (c) six waves reduced to four waves due to the phase mismatch.

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Stability of the FWM cascades for six waves is then evaluated. Figure 4(a) shows the recorded output spectra of the FWM cascades that are scanned at two minutes interval for half an hour at EDFA output power of 20.57 dBm. The pumps, which are the two strongest waves in the spectra are tuned at 1552.97 nm and 1564.87 nm. As a result of FWM cascades, four new waves having OSNR larger than 10 dB appear at 1529.76 nm, 1541.24 nm, 1576.92 nm and 1589.15 nm. Noise bump in the spectra results from the ASE noise of EDFA, which can be considerably reduced by spectral filtering. For a closer look at the stability of the waves, we then plotted the peak power fluctuation of each wave in Fig. 4(b). It can be observed that the peak power fluctuations are not significant as their variation is less than 1 dB. This demonstrates the stability of the FWM cascades.

 

Fig. 4 (a) Output spectra of FWM cascades for six waves case when pumps were tuned at 1552.97 nm and 1564.87 nm at 2 minutes interval for half an hour and (b) peak power fluctuations of the FWM cascades during the scanning.

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4. Conclusion

In this paper, a method of generating multi-wavelength laser based on FWM cascades technique without the use of external tunable laser sources as pumps is experimentally demonstrated. The pumps for cascading effect are provided by a dual wavelength EDFL that is stabilized by the FWM mechanism within the 500 m HNLF. Phase or intensity modulation is not needed in this proposed scheme as 0.23 nm FWHM bandwidth of TBFs makes the linewidth of the pumps wide enough for SBS suppression. Number of waves for the output can be controlled by adjusting EDFA output power. As the EDFA power increases, the number of waves produced becomes larger. At EDFA output power of 20.57 dBm, six waves having OSNR larger than 10 dB appear at the output. The six waves are stable with peak power fluctuations of less than 1 dB for half an hour at 2 minutes interval. The wavelength spacing tunability for six waves case is also investigated. The minimum and maximum wavelength spacings are 1.03 nm and 11.31 nm respectively. The maximum spacing can be improved further if we can engineer the dispersion property and length of HNLF such that phase matching of waves spans over a wider bandwidth.