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Low-noise optical frequency comb from diode-pumped solid-state laser

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Optical frequency combs from modelocked lasers have revolutionized numerous research areas in time-frequency and optical metrology in the last decade by providing a direct and phase-coherent link between microwave and optical frequencies.

A major research area at LTF is the characterization, optimization and stabilization of novel frequency comb technologies. Diode-pumped solid-state lasers (DPSSLs) constitute such a novel technology that combines many benefits of the two most commonly used types of frequency comb today, i.e., frequency combs based on Ti:Sapphire lasers and fiber lasers. Similar to Ti:Sapphire lasers, the high Q-factor of the DPSSL optical resonator leads to low quantum noise operation. In addition, DPSSLs benefit from convenient diode-pumping like fibre lasers, which provides a simple way to control and stabilize the carrier-envelope offset (CEO) frequency via pump current modulation.

In collaboration with the group of Prof. Ursula Keller at ETH Zurich, we developed, optimized and fully stabilized the first DPSSL frequency comb in the 1.5 µm spectral range. The Er:Yb:glass laser oscillator (ERGO) depicted in Figure 1a has 75 MHz repetition rate and showed a record narrow-linewidth free-running CEO beat for a frequency comb at 1.5 µm with less than 4 kHz full width at half maximum. This enabled a tight lock to be achieved by pump current modulation with a feedback bandwidth of only 5.5 kHz. With such a low bandwidth, a residual integrated phase noise of 720 mrad was demonstrated, leading to a 20-fold improvement in the fractional frequency stability of the 20 MHz CEO beat compared to a commercial Er:fiber comb stabilized with the same electronics (Figure 1b).
   
  

Figure 1:(a) Schematic view of the ERGO DPSSL. (b) Allan deviation of the CEO frequency in the ERGO comb and in a commercial self-referenced Er-fiber comb for comparison.

The physical mechanisms leading to the self-referencing of the ERGO DPSSL comb, i.e., the stabilization of the comb CEO frequency f CEO, have been studied. We showed for the first time that a high signal-to-noise ratio and a low-noise CEO beat are not sufficient indicators for the feasibility of comb self-referencing, but that the dynamics of the CEO frequency also plays a dominant role, i.e., how the CEO frequency changes in amplitude and phase with a modulation of the pump current.

In the ERGO comb, the presence of a reversal point in the tuning curve of fCEO as a function of the pump current (Figure 2a) led to the impossibility to self-reference the comb at pump currents located on one side of the reversal point, whereas stabilization was straightforward on the other side despite a lower signal-to-noise ratio of the CEO beat. We showed that the important difference observed in the dynamic response of f CEO for pump current modulation was responsible for the impossibility to stabilize f CEO in some operating conditions.

Indeed, the transfer function of f CEO for pump current modulation shows a low-pass filter behaviour with no significant phase shift up to ~1 kHz at pump currents where stabilization is straightforward, whereas a large phase shift in the CEO-beat dynamic response is observed already at a very low frequency on the other side of the reversal point where stabilization is not possible. This result is not only important for the ERGO DPSSL considered here, but may also have a significant impact for the future self-referencing of novel comb technologies.

 

Figure 2: (a): Static tuning curve of the CEO frequency f CEO with respect to the pump current showing the presence of a reversal point. (b): amplitude (top) and phase (bottom) of the dynamic response of fCEO to pump current modulation measured at different pump currents: I p = I low below the reversal point (green curves), I p = I high above the reversal point (red curves) and I p = I reversal at the reversal point (orange curves).
 

Relevant publications :

  1. S. Schilt, N. Bucalovic, V. Dolgovskiy, C. Schori, M. C. Stumpf, G. Di Domenico, S. Pekarek, A. E. H. Oehler, T. Südmeyer, U. Keller, P. Thomann, Fully stabilized optical frequency comb with sub-radian CEO phase noise from a SESAM modelocked 1.5-μm solid-state laser, Opt. Express 19(24), 24171-24181 (2011) PDF
  2. S. Schilt, V. Dolgovskiy, N. Bucalovic, C. Schori, M. C. Stumpf, G. Di Domenico, S. Pekarek, A. E. H. Oehler, T. Südmeyer, U. Keller, P. Thomann, Noise properties of an optical frequency comb from a SESAM-modelocked 1.5 µm solid-state laser stabilized to the 10-13 level, Appl. Phys. B 109 (3), 391-402 (2012) PDF
  3. V. Dolgovskiy, N. Bucalovic, P. Thomann, C. Schori, G. Di Domenico, S. Schilt, Cross-influence between the two servo-loops of a fully-stabilized Er:fiber optical frequency comb, J. Opt. Soc. America B 29 (10), 2944-2957 (2012) PDF
  4. N. Bucalovic, V. Dolgovskiy, M.C. Stumpf, C. Schori, G. Di Domenico, U. Keller, S. Schilt, T. Südmeyer, Effect of the Carrier-Envelope-Offset Dynamics on the Stabilization of a Diode-Pumped Solid-State Frequency Comb, Opt. Letters 37 (21), 4428-4430 (2012) PDF