Quantum Optics Lab

Treutlein Group

 

Interference filter lasers: Lower noise for precision applications

 

by Andreas Jöckel, 2016

Introduction - why low noise?

In optomechanics any technical laser noise at the mechanical oscillators frequency leads to heating and therefore additional decoherence of the oscillator. This can prevent reaching the quantum ground state and limits the lifetime and fidelity of quantum operations performed on the system. Laser noise also affects any interferometric measurement and optical cavity systems, hence it is beneficial or even necessary to decrease the noise as far as possible.

In our experiment, a laser wavelength of 780nm is required, where only a few laser sources are available, including titanium:sapphire (Ti:Sa) laser and diode lasers. Diode lasers are the much more affordable option, but do not offer the best noise performance. I improved these lasers now to be en pair with the frequency noise performance of a Ti:Sa laser and surpassing it in terms of intensity noise.

Laser noise

Here will be an introduction to laser noise in the future. Meanwhile please consult my thesis (chapter 1.2.4 and 4.2.4) for background information regarding laser noise.

The graphs on the right show intensity and frequency noise of various lasers at 780nm measured using an asymmetric Michelson interferometer with 60m path difference and a detection limit of about 0.1Hz/rtHz. The intensity noise is measured at about 15mW-20mW of laser power using a simple large area photodiode and a low-noise current amplifier. At this power level, shotnoise is around -163dBc/Hz.

Diode lasers are limited in intensity noise by the current noise, which can be reduced by proper low-pass filtering of the source. The laserheads have a 500Hz LC low-pass integrated, who's response corresponds to the noise drop off in the graph. The Ti:Sa laser shows worse performance up to the relaxation-oscillation peak at 1MHz.

The frequency noise on the right shows a different behavior. Diode lasers usually have very short low finesse cavities, which results in a broader spectrum and therefore high frequency noise. The developed interference-filter lasers have longer and stiffer cavities and show superior performance compared to the shown grating-stabilized diode laser while maintaining tuning capability. A special low noise version sacrifices the mode-hop free tuning for best noise performance, which allows it to compete with the Ti:Sa laser at a much lower price.

 

Operating principle

Classically, an external cavity diode laser is built by feeding the -1st order reflection of a grating back into the laser diode. This results in a frequency filtering, as the grating angle determines the frequency of the retro-reflected light. The operating principle is illustrated in the figure on the right. The grating based lasers offer large mode-hop free tuning, especially in the similar Littman-Metcalf configuration. An inherent disadvantage is the movable grating, which reduces the long term stability of the laser cavity. The wavelength dependent output angle can be compensated by an additional mirror, which is fixed to the grating.

Interference filter based diode lasers have emerged in the last years as narrow tunable interference filters with high transmission became available. They have transmissions up to 98% with a spectral FWHM of 0.35nm at 780nm laser wavelength. Their transmission frequency can be tuned by tilting the filter. The per-angle-tuning is lower compared to a grating and drifts or vibrations of the filter do not influence the total cavity length. Therefore, the laser cavity can be constructed as a monolithic system with the piezo at the so called cat-eye end reflector as a fine-tuning element. The cat-eye construction consists of a lens that focuses the laser beam onto the endmirror, thus ensuring a 180° reflection angle. However, the transverse position of the element has to be aligned to ensure an optimal mode overlap with the incident beam.

The benefits of this construction are the less sensitive alignment and resulting better long term stability as well as the insensitivity of the output mode to coarse wavelength tuning. In addition, depending on the output mirror reflectivity and cavity length, this laser can offer significantly lower noise. In the following two laser constructions are introduced where one focuses on compactness and large mode-hop free tuning as a general purpose laser and the other on lowest possible noise, which results in narrow linewidth and low spectral noise, comparable to Ti:Sa lasers.

 

Ultra-low noise laser

This laser was designed for optomechanics experiments, which are extremely sensitive to laser noise. There, the noise power at the frequency of the mechanical oscillator is determining the optomechanical cooling performance. This noise has been reduced by extending the laser cavity to about 2.5m while keeping a small footprint of the cavity housing. The total linewidth also benefits from this and becomes very narrow. Remaining fluctuations are dominated by low frequency mechanical modes and could be compensated by locking to a stable cavity, which should result in linewidths <<1kHz. A picture of the laser is shown below, next to the specifications.

 
 
 
Property Value
Wavelength 778..782nm
Power 30mW
Piezo tuning 100MHz
Linewidth <20kHz@2.3ms
Intensity noise @100kHz -153dBc/Hz
Frequency noise @200kHz 1Hz/rtHz
DC-Modulation DC..100MHz
AC-Modulation 80..600MHz
Footprint 10x30cm
All properties are typical values only and depend on the laser diode and optics properties.
 

General purpose laser

 

The second version is a general purpose laser with reduced cavity length and focus on usability and modulation/tuning capabilities. It has integrated cylindrical lens pair for beam shaping and single stage isolator, coarse wavelength tuning form outside, large mode-hop free tuning and fast modulation. The specifications are shown below next to a picture of the laser.

 
 
 
Property Value
Wavelength 775-790nm
Power 95mW
Piezo tuning 15-30GHz
... without current feed-forward 2GHz
Linewidth <20kHz@2.3ms
DC-Modulation 4mA/V, DC..100MHz
AC-Modulation 80..600MHz
Piezo-Tuning -25..150V, 1V/GHz
Footprint 10x25cm
All properties are typical values only and depend on the laser diode and optics properties.
 

Sample measurements of laser intensity noise for various lasers.

Sample measurements of laser frequency noise for various lasers.