Femtosecond laser delivers breakthrough performance

A Ti:Sa laser that emits sub-50 fs pulses at repetition rates of 10 GHz offers unique advantages for applications in spectroscopy.

Scientists in Germany and the US have built a passively modelocked Ti:Sa laser that ahieves an unbeatable combination of high bandwidth, high average power and repetition rates of up to 10 GHz. The laser, which delivers pulses of down to 42 fs, is intended to increase the signal-to-noise ratio of spectroscopic measurements that require a laser frequency comb, and could also be used in the development of optical clocks.

"The combination of high repetition rate and large fractional bandwidth is a measure for the merit of a frequency comb for spectroscopy purposes," said Albrecht Bartels of Gigaoptics, a German company that already markets femtosecond lasers operating at 1 GHz and 5 GHz. "The fractional bandwidth is typical for other Ti:Sa lasers, but very large compared with other 10 GHz sources — which usually deliver picosecond pulses."

The crucial advantage of such high repetition rates is that for the first time it allows the individual modes of the femtosecond laser — in other words, the "teeth" of the frequency comb — to be separated with a simple grating spectrometer.

"The spacing between the frequency comb modes depends on the repetition rate," explained Bartels. "Most applications of femtosecond laser frequency combs only require a single or a few specific modes out of the many available. Now we are able to isolate these modes and individually direct them to an experiment, while unwanted modes that only create additional noise are excluded."

According to Bartels, the laser supports around 500 modes, each separated by precisely 10 GHz. And because the spacing between the modes is larger than at lower repetition rates, the output power from the Ti:Sa laser is spread between fewer modes.

As a result, each mode delivers power levels of more than 1 mW, which is more than enough for most spectroscopic applications. "Some applications require only nanowatts per mode, but more power means more signal-to-noise ratio and thus quicker measurements."

The new laser design, which was unveiled in a post-deadline paper at the Conference for Lasers and Electro-Optics (CLEO) in May, was developed by Bartels in collaboration with researchers at the University of Konstanz in Germany and the US National Institute of Standards and Technology (NIST) in Boulder, Colorado. Bartels told optics.org that a commercial version of the laser is due to be launched within the next six months.

According to Bartels, the key parameter for achieving high repetition rates is the peak intracavity intensity, which is increased by tightly focusing the pump laser to a 10 µm spot within the Ti:Sa laser. The ring cavity design also supports higher repetition rates for a given number of cavity mirrors, in this case a minimum of four.

The use of a Ti:Sa crystal also ensures a broad gain bandwidth, as well as efficient pump light absorption and high gain over a short length — which is essential to achieve the 10 GHz repetition rate. The output wavelength of 783 nm is also useful for many applications.

"Most importantly, it matches the resonances of useful atomic systems, such as rubidium and caesium, which will allow the 10 GHz frequency comb to be locked to such atomic references for precision spectroscopy purposes," said Bartels.

The next stage, says Bartels, is for his co-workers at NIST to demonstrate the use of the laser in applications such as direct frequency comb spectroscopy, waveform generation, and astronomical spectrograph calibration. Further work on the mechanical packaging of the laser will also be needed before a commercial device can be released.