Long-Term Stabilization Based on Deviation-Frequency Locking

In order to achieve the long-term stabilization of asynchronous mode-locked Er-fiber soliton lasers successfully, both mechanisms of asynchronous mode-locking and hybrid mode-locking should be maintained during the stabilization operation [6]. The former mechanism relies on maintaining a suitable deviation frequency of 15–40 kHz between the modulation frequency and the cavity harmonic frequency. For the latter mechanism, the optical polarization evolution along the laser cavity should not be perturbed by the cavity length control unit. In this way, polarization additive mode-locking and additive pulse limiting can continue to maintain the same effects of pulse formation and pulse energy equalization. To meet the first requirement of keeping a suitable deviation frequency, we feedback-control the cavity length by a PZT to lock the deviation frequency at a suitable frequency, i.e., 25 kHz. As to the second requirement, since we do not observe obvious polarization changes when the cavity length is detuned by the PZT, it is automatically satisfied. The shift of the deviation frequency can be obtained by using a low-pass filter to extract the first peak closest to DC from the frequency sidebands near DC.

Our actual experiment to demonstrate long-term stabilization is described as follows. The asynchronously mode-locked fiber laser is fixed on the aluminum plate with a temperature controller to reduce the environmental temperature change. A PZT wound with the single-mode fiber is put in the laser cavity to adjust the cavity length.

As shown in Fig. 4-3, the feedback control loop composes a sharp cutoff (135dB/Octave) low-pass filter, an amplifier, a frequency counter, a computer, and a high voltage amplifier to drive the PZT. A small fraction of the laser output is detected by the photodiode. The low-pass filter can remove all the sideband peaks except for the first frequency peak closest to DC. The frequency counter (Agilent 53181A) can measure the deviation frequency and sends its digital output to the computer. The computer will then process the signal with suitable signal processing methods and then send the frequency error signal to the PZT driver to lock the deviation frequency at 25 kHz. The maximum amount of the frequency shift that can be produced by our PZT unit is about ±25 kHz, which seems to be enough for long-term stabilization as long as the environmental temperature fluctuations are reduced to be small enough by the temperature controller. When the fiber laser is operating without the stabilization scheme, the typical deviation frequency is about ±4 kHz in 7 minutes as shown in Fig. 4-4 Experimentally, a deviation frequency shift larger than 10 kHz will severely affect the stability of asynchronous mode-locking. Long-term stable operation is achieved when the stabilization scheme is turned on, as is shown in Fig. 4-4. The deviation frequency shift can be controlled to be within ±300 Hz, limited by our frequency counter unit. Figure 4-5 shows the stabilized 10 GHz pulse train measured from a fast sampling oscilloscope.

The main advantage of this stabilization scheme is that it provides a simple and economic approach to stabilize asynchronously mode-locked fiber lasers. The high-speed electronics are not required in the feedback control loop and it is much easier to deal with the electronics in the kHz range. Furthermore, the same feedback control unit is suitable for even higher modulation frequencies. This is because the suitable deviation frequency always remains within the range of 15–40 kHz, despite the change of the pulse repetition rate.

4.5 Summary

To summarize, without using high speed RF feedback electronics, the long–term stabilization of a 10 GHz 0.8 ps asynchronously mode-locked Er-fiber soliton laser has been proposed and demonstrated for the first time by controlling the cavity length to lock the deviation frequency at 25 kHz. The stabilization scheme is simple and economic, since only electronics in the kHz range are required for long-term stabilizing the mode-locked fiber laser with a 10 GHz repetition rate. Moreover, the same low frequency feedback control unit is suitable for other modulation frequencies, even when the pulse repetition rate is raised up to 40 GHz or more. Based on this stabilization scheme, stable sub-ps pulses with high repetition rates can be stably obtained from asynchronously mode-locked Er-fiber soliton lasers for real applications

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[15] http://www.kemo.com/.

(a)

(b)

Fig. 4-1 RF spectra near 10 GHz. (a) Span of 50 MHz, SMSR > 70 dB (b) Span of 500 kHz, deviation frequency δf of 25 kHz.

(a)

(b)

(c)

Fig. 4-2 (a) Electronic frequency sidebands of laser output near DC (b) Electronics of deviation-frequency extraction (c) Electronic frequency spectrum after the lowpass filter (inset, signal in the time domain).

Fig. 4-3 Schematic of the mode-locked Er-fiber laser and the feedback control. BPF, band-pass filter; PD, photodiode; DSF, dispersion shift fiber.

Fig. 4-4 Long-term stabilization and deviation frequency shift. Green curve: without the stabilization scheme, upper axis; blue curve: with the stabilization scheme, lower axis.

Fig. 4-5 10 GHz pulse train measured from a fast sampling oscilloscope.

Chapter 5