Tuning with finesse

29 August 2013

Members of the Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory

Members of the Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory 

Comb lines can be tuned by cavity length control

Comb lines can be tuned by cavity length control

Researchers at the Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory in the US have reported tuning of the frequency envelope of a broadband comb source at 1.5-µm, generated from a semiconductor-based mode-locked ring laser with an intracavity high finesse Fabry-Perot etalon.  

Broadband, stable spectral comb sources find use in a number of applications, such as multi-wavelength light sources, multi-channel dense WDM transmission, microwave photonic filters or signal processors and high speed optical communications. Wavelength tuning of such sources will lend flexibility in the device’s operation and may reduce the cost of system implementation by allowing the use of cheap passive and fixed counterpart components.

Ringing the changes

In a ring laser, the cavity (resonator) is formed by a ring structure (closed loop), instead of two reflecting mirrors. Professor Dongsun Seo explained that “the cavity includes: a gain medium for light gain; an isolator for unidirectional operation and; a shutter for active mode-locking.” For stable mode-locking, the number of lasing modes can be limited by using a band-pass filter inside the cavity. However, band-pass filters cannot suppress super-mode beating noise, and “to suppress the noise,” explained Seo, “periodic spectral filtering of individual lasing modes (comb lines) is essential. Currently, a Fabry-Perot etalon (FPE) is the only solution for periodic filtering.”  

A Fabry-Perot etalon is made using a transparent plate with two reflecting surfaces or using two reflecting mirrors. Its transmission spectrum as a function of frequency exhibits perfect transmission peaks corresponding to resonances of the etalon, “and this property,” said Seo, “means they can be used as periodic band-pass filters with periods corresponding to the etalon resonance frequencies.”

Pushing the envelope

In their Letter, the Purdue team have studied one of the more cost-effective schemes for wideband flat optical frequency comb generation: a semiconductor-based mode-locked ring laser with an intra-cavity high-finesse FPE. “When developing a mode-locked laser, it is important to understand the comb line selection mechanism,” said Seo, “so we studied the comb line selection mechanism based on the optical comb frequency alignment with respect to the FPE transmission peaks.” They found that, by detuning the alignments, they could apply frequency dependent and controllable loss to the individual comb lines. This controllable loss led to flexible operation of the laser, including effective spectral bandwidth adjustment or wavelength tuning.

This envelope tuning capability (particularly of a large number of flat comb lines) gives great flexibility in a number of applications, such as arbitrary wavelength region selection of a multi-wavelength source for WDM communications. Controllable operation of the laser enables the user to determine the allowable margins of the key parameters for implementing a compact, practical comb source. In addition, the relationship between the spectral envelope tuning and slight detuning of the repetition rate and cavity length may also provide a new principle applicable to measuring the free spectral range of an FPE.

Tuning the tuning

To develop a commercialised comb source with such tuning, compact, temperature-stabilised components is essential for modern communication devices. Custom ordered components are available now, but the cost is a big issue. Furthermore, explained Seo, “even if all components and setup are well stabilised, residual perturbation may disturb the condition for spectral envelope translation, especially at the moment of tuning.” Therefore, the next major challenge will be to achieve accurate and repeatable automatic translation to a desired comb wavelength region.

Professor Seo broke this down further: “Firstly, we are trying to find allowable system margins for stable operation of the laser - we may find a cost effective way to implement the laser - and secondly, we are looking for a method to solve the challenging issue discussed previously, perhaps using external light injection. Finally, we will apply the source to measure comb line stability of other (relatively unstable) pulse sources.”

Research in the Purdue University Ultrafast Optics and Optical Fiber Communications Laboratory focuses broadly on ultrashort pulse and broadband optical signal processing. As Seo explained, “Prof. Weiner's group [collaborators in this work] is especially well known for its work on ultrashort optical pulse arbitrary waveform generation.  Applications include lightwave communications, radio-frequency photonics, and ultrafast optical science.”  More specifically, the Purdue laboratory are currently researching high repetition rate optical frequency combs, photonic generation of arbitrary radio-frequency waveforms and photonic implementations of programmable radio-frequency filters, nonlinear optics in microresonators including comb generation, applications of pulse shaping to quantum optics, and compensation of ultrabroadband wireless propagation under strong scattering.

Further reading

This article is based on the Letter: Tuning power spectrum of a semiconductor and intracavity-etalon based mode locked laser via detuning (new window).

Purdue University:https://engineering.purdue.edu/~fsoptics/ (new window)

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, Volume 49, Issue 25

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