Fast opto-optical modulator based on SESAM optical pumping

key members:

A carrier-envelope offset (CEO) phase actuator with sufficiently high modulation bandwidth is a key element for self-referencing of a frequency comb. The modulation of the optical pump power of the laser gain material is currently the mostly used CEO-locking method. However, its modulation bandwidth is limited by the upper-state lifetime of the gain. For many important diode-pumped Yb and Er-materials, the upper-state lifetime is in the millisecond regime, restricting the feedback bandwidth to the kHz-regime. By using direct intra-cavity modulation, a larger bandwidth can be achieved. The use of intra-cavity acousto-optical modulators (AOMs) and electro-optical modulators (EOMs) allowed increasing the modulation speed and reducing CEO noise levels. However, inserting an additional modulator into the laser cavity is often a challenge, for example in multi-GHz repetition rate lasers where geometrical constraints limit such an approach. Intra-cavity AOMs and EOMs can exhibit high nonlinearity, which can destabilize or prevent modelocking of the laser, or introduce thermal lensing in high power lasers. Very recently, a graphene EOM with MHz modulation bandwidth was used to stabilize the CEO frequency in a Tm-fiber similariton oscillator. However, intra-cavity graphene EOMs do not appear well-suited for the stabilization of diode-pumped solid-state lasers (DPSSLs) due to their relatively high non-saturable losses (currently 5%), which is comparable or higher than the typical output coupling, and the small aperture limiting the achievable intra-cavity power levels. 

At LTF, we developed a new method for CEO frequency control of ultrafast lasers that combines high feedback bandwidth with low loss, low nonlinearity, and low dispersion. A semiconductor saturable-absorber mirror (SESAM) inside the modelocked laser is optically-pumped with a continuous-wave (cw) laser. In this way, the SESAM acts as intra-cavity opto-optical modulator (OOM). The optical power of the cw laser corresponds to a high-bandwidth modulation channel for CEO frequency control. We experimentally verified this method for a femtosecond Er:Yb:glass oscillator ( ERGO [1-2]), in which one SESAM is, at the same time, used for modelocking and as intra-cavity OOM for achieving a tight CEO-lock. This laser can also be CEO-locked in the usual scheme, in which the laser pump current is modulated. We compared the performance of the stabilization for gain and SESAM-modulation by measuring the CEO transfer function, frequency noise power spectral density, and Allan deviation for integration times up to 1000 s (see Figures 1 and 2). In the case of the gain modulation, the millisecond upper-state life time of the Er:Yb:glass limits the achievable CEO-control bandwidth to <10 kHz. The feedback bandwidth of the SESAM OOM was more than a factor of 10 higher than the standard pump modulation. The residual integrated phase noise (1 Hz – 100 kHz) of the ~20-MHz CEO-beat was improved by more than an order of magnitude (from 720 mrad to less than 65 mrad), and the fractional frequency stability by a factor of 4 (from 1∙10-8 to 2.5∙10-9 at 1 s).

These first results highlight the large potential for CEO-locking based on optically-pumped SESAMs. It combines low dispersion, low thermal lensing, and low losses, and should therefore be easy to implement even for high power ultrafast thin disk lasers. Such performance will be highly attractive for applications such as intra-cavity multi-MHz high-harmonic generation (HHG) providing coherent and frequency-stabilized VUV/XUV radiation. Considering the short recombination time of semiconductor absorbers, there does not seem to be any practical limit for the modulation bandwidth, and we expect that our technology will enable low-noise CEO-stabilization of new fiber lasers and DPSSLs which are not well-suited for the classical CEO-stabilization methods.


Figure 1:  Comparison between the performances of the stabilized CEO beat obtained for standard pump current modulation (blue) and the new SESAM-pumping method (red) in terms of  RF spectrum (left) and frequency  noise power spectral density (right) of the CEO beat.

Figure 2:  Comparison of the fractional frequency stability (Allan deviation) of the stabilized CEO beat obtained for standard pump current modulation (blue) and the new SESAM-pumping method (red).

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. M. Hoffmann, S. Schilt, T. Südmeyer, CEO stabilization of a femtosecond laser using a SESAM as fast opto-optical modulator, Optics Express 21 (24), 30054-30064 (2013) PDF