1. Introduction

Ultra short pulse trains with high-repetition-rate at multiple wavelengths may be required in fiber optical sensing and high-bit-rate optical communications. Many Erbium-doped fiber (EDF) laser configurations have been explored to realize these multiple wavelength pulse trains. However, Due to the large homogeneous linewidth broadening of EDF gain medium at room temperature, it’s often difficult to generate multi-wavelength pulses by mode locking at close wavelengths. In addition, the stabilization of simultaneous mode locking for multiple channels still remains a problem. To overcome these limitations, several technologies have been reported [1–4], in ref. [1], Hayashi et al obtained 16 wavelengths laser source by cooling the erbium-doped fiber with liquid nitrogen to reduce its homogeneous linewidth. Some researchers also achieved multi wavelength pulse trains by introducing cascaded fiber Bragg gratings (FBGs) in the laser cavity [2], in this configuration, however, the simultaneous mode locking can’t be realized because the cavity lengths are different. In [3], instead of using single EDFA, parallel amplifiers were adopted in cavities. Nevertheless, due to the fluctuation of laser cavity and dual-mode competition, stability was limited.

In this paper, using a modulation frequency f _m=10 GHz, we achieve stable optical pulse trains at the repetition rate of 10 and 40 GHz with pulse widths of 8ps and 5ps respectively. The instability has been reduced considerably by using an intracavity semiconductor optical amplifier (SOA) to reduce the gain homogeneity and a phase locked loop (PLL) for cavity length stabilization..

2. Principle of stabilization

The principle of two-wavelength mode locking at different repetition rates is as follows. The harmonic mode-locking happens when the modulation frequency f_m = N ∙ f _c1, where N represents the harmonic number, which is an integer, f_m is the modulation frequency provided by the VCO, and f _c1 the fundamental cavity round trip frequency for channel 1, (one of the lasing wavelengths). These quantities are given by:

Here c represents the speed of light in vacuum, n_eff is the effective refractive index of the ring cavity and L₁ is the total length of the ring cavity. Since L₁ is more than 50 m long in this experiment, the fundamental cavity frequency f _c1 should be only several MHz. Figure 2(a) shows the 10 GHz pulse train generated in channel 1, corresponding to λ ₁ = 1546nm. It shows a pulse width of 8ps. For the second wavelength, the rational harmonic mode locking is observed when modulation frequency f_m is set at $(M+\frac{1}{p})$ multiple of the fundamental cavity frequency f _c2, i.e.

where, the rational harmonic number p is an integer, and L₂ is the cavity length with respect to the second wavelength. L₂ is precisely adjusted (using the delay line) so that p=4 satisfies Eq. (2) for the second wavelength, from Eqs. (1) and (2), (assuming same n_eff)

or

where ∆L = L ₂ - L ₁, From (1), we have

Here, the total cavity length is provided in terms of cavity round-trip time (ps). Using Eqs. (4) and (5) for p=4,

Thus, once the 10GHz-40GHz operation is mode locked, the round trip time difference between two cavities should be (100∙(M - N) + 25)ps.

In practical applications, the stability of the pulse trains is of vital importance. Generally, for the multiple wavelength pulses generated by mode locking, two main factors contribute to pulse instability: 1) Cavity length drift due to thermal fluctuation. 2) Competition between multiple modes in the laser cavity.

As is known, in the 10GHz-40GHz two wavelength mode locking operation, 10GHz pulse generation can be realized in the ring cavity configuration when the modulation frequency f_m is adjusted with high precision to nf_c, while 40GHz is generated in the meantime by rational harmonic mode locking. As is stated above, two cavity round trip time should differ by ~ (100∙(M - N) +25)ps in order to keep the mode locking conditions for both wavelengths, however, the cavity lengths may drift due to the environmental thermal influence. For this reason, we introduce phase locked loop (PLL) in the cavity to maintain the mode locking conditions. The key components of the PLL include a phase shifter, a 10 GHz mixer, a phase locked loop controller and a voltage-controlled oscillator (VCO). The mixer detects the phase difference between the photodiode output and the VCO, and then sends it back to the PLL controller, which will produce an output of v_bias - ∫v_mixer dt. Thus through negative feedback, the circuit continuously adjusts the modulation frequency to the mode locking frequency, assuring long-term stability of 10GHz pulse train in the operation.

Once 10GHz pulse train is mode locked through PLL, 40GHz pulses can automatically be mode locked as long as the cavity length difference between two channels (∆L) is reasonably short. To understand that, we assume both N and M will no longer change during the 10GHz mode locking operation. As is already known, ∆L, should be (100 ∙(M - N) + 25)ps for 10GHz-40GHz two-wavelength operations. While from equation (4), the corresponding ∆L, for 10GHz-30GHz and 10GHz-50GHz operation should be (100∙(M - N) + 25)ps and (100 ∙ (M - N) + 25)ps respectively. Which means, in order to break up the 40 GHz rational harmonic mode locking conditions, the drift of ∆L, should at least be 5ps, or ~1mm long. Since for the silica fiber, the ring cavity length fluctuates by 0.011 mm/°C·m [5], ∆L, must be as long as 100m to produce 1mm thermal drift under normal room temperature. While in this work, the length difference between two cavities (∆L) is less than 1m, therefore, 40GHz pulse train can also be mode locked despite the tiny thermal drift of ∆L.

The broad gain spectrum characteristics of the SOA may make the two-wavelength operation more stable at room temperature. In this work, we use the low saturation effect of the SOA, which stabilizes any fluctuations and results in nearly equal amplitudes. In the two-wavelength mode-locking scheme, due to the mode-competition effect as observed previously [6], two pulses with different wavelengths will compete for the available gain in the Er-doped fiber. Consequently, any oscillation of one pulse will possibly suppress that of the other [7]. Since the inhomogeneous broadening property of SOA makes it easier to generate multiple lasing modes, by introducing SOA in the laser cavity, thereby lessening the gain competition between two wavelengths, the intensities of two pulse trains will finally reach equilibrium. However, we note that, since the population inversion factor of SOA is larger than that of EDFA, the insertion of SOA into the cavity may increase the laser system noise figure.

3. Experimental results

The schematic of a ring fiber laser with an SOA in the optical cavity is shown in Fig. 1. In this experimental set up, the EDFA is pumped by a 980nm pump laser, with the pumping current of 200mA. The SOA is driven by a current source. A pair of multiplier/demultiplier (MUX/DMUX) is employed in the cavity as the wavelength selective element, which has 4 channels with the bandwidth of 1.5 nm each. To overcome the homogeneous linewidth broadening limitation, we use the 1^st channel (1558nm) and the 4^th (1546nm) channel of MUX/DMUX as working channels. In order to vary the cavity length (which determines the round-trip time of the optical pulse in the cavity) of both channels, we insert a delay line in each cavity. By carefully tuning the delay line 1, we vary the total optical cavity of 1558nm channel until 10GHz pulse train is achieved by harmonic mode locking. Then we realize rational harmonic mode locking of 40GHz pulse train in the 1546nm channel by tuning delay line 2. Once the unstabilized 10GHz-40Ghz pulse trains have been obtained in the cavities, we close the optical-electrical phase locked loop (PLL). A 10/90 coupler is placed after the MUX.

 

Fig. 1. Schematic of two-wavelength fiber ring laser.

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Fig. 2. Pulse trains observed by autocorrelator. (a) 40GHz pulse train with pulse width of 5ps (b) 10GHz pulse train with pulse width of 8ps

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The loss budget for the optical ring cavity are: 7 dB total for the modulator (including polarizer and isolator), 9.5 dB for the MUX/DEMUX system, 3 dB net gain for SOA, ~ 1 dB for coupler and other losses. Thus the total loss is ~ 14.5 dB which equals the EDFA gain. The LiNbO₃ electro optical modulator is driven by a modulation signal, which is controlled by the 10GHz voltage-controlled oscillator (VCO). The output of the VCO is amplified to 31 dBm. In this work, the pulse shape is observed by using autocorrelator.

Figures 2(a) and (b) show autocorrelator traces of the 10GHz and 40GHz pulse trains, The 10 GHz pulse train is generated at the drive frequency of the modulator and the 40 GHz pulse train is at 4 times the drive frequency. The measured pulse widths are 5ps and 8ps respectively.

 

Fig. 3. Spectrum of the pulses at wavelength of 1546 nm (40 GHz) and 1558nm (10 GHz) respectively.

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Figure 3 illustrates the optical spectrum of the pulse trains. The width of the optical spectrum of 10 GHz pulse train is narrower than that of the 40GHz pulse train. As we have shown above, since the repetition rates are determined by the rational harmonic number p, we may also generate two sets of 40 GHz pulse trains by satisfying Eq. (2) with p=4 for both wavelengths. Therefore, by carefully tuning each delay lines to vary the cavity lengths, we achieve 40GHz-40GHz two-color operation with the autocorrelator traces shown in Fig. 4

 

Fig. 4. 40GHz Pulse trains with wavelengh of (Left) 1546nm and (Right) 1558nm.

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Figure 5 demonstrates the measurements of intensity of 40GHz pulse train vs. time. For the two cases: (i) PLL and SOA in the fiber laser configuration and (ii) without PLL and SOA. Stable performance is obtained with the phase locked loop and SOA in the cavity. The measured timing jitter of the 10GHz and 40GHz pulse trains, are 130fs and 145fs respectively. The amplitude equalized pulses were a result of using the PLL and the inherent stabilization behavior of SOA (due to its saturation characteristics) in the optical cavity. The operation of the mode locked ring laser is stabilized using the PLL, in its absence the mode-locking is not stable. The result is shown in Fig. 5.

 

Fig. 5. Stability of pulse train (a) intensity and pulse form of 40 GHz pulse train with SOA and PLL in the fiber ring. (b) Intensity and pulse form of 40 GHz pulse train without SOA and PLL in the fiber ring.

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The two wavelengths were mode-locked at different frequencies in this experiment intentionally using delay lines. The mode locking at one wavelength occurs by rational harmonic mode locking and for the second it occurs by harmonic mode locking (40 GHz, 10 GHz case) or also by rational harmonic mode locking (40 GHz, 40 GHz case). This necessitates different delay lines for the two wavelengths. The operation at a given wavelength or multiple wavelength mode locked operation can be achieved using a similar technique and using optical filters in each of the several delay lines. Wavelength tunable operation is also feasible.

4. Summary

We have demonstrated simultaneous stabilized operation of a mode locked ring fiber laser at two wavelengths. At one of the wavelengths the mode locking operation is at 10 GHz and it is at 40 GHz at the second wavelength. The laser has an intracavity LiNbO3 modulator driven by 10 GHz VCO. The 40 GHz pulses are obtained by rational harmonic mode locking. Pulses with widths in 5 to 8 ps range are obtained. By using PLL, we simultaneously stabilize the 10GHz and 40GHz pulse trains. SOA is inserted in the cavity to lessen the mode competition effect. Mode locking at 40 GHz for both wavelengths has also been demonstrated. We show that the stability of pulse trains is considerably improved using a phase locked loop.