The high short-term stability of the iodine reference can be distributed to any CW laser whose frequency is within the comb spectrum using the OFC as a transfer oscillator.
The PLL based on the beat notes between the CW laser and the OFC can be used to narrow the optical linewidth of the CW laser. The required wavelength for electronic excitation of the OH at 308 nm is outside the comb spectrum, so the spectroscopy laser in this thesis is based on a dye laser at 616 nm (Sirah GmbH Matisse 2 DR) which is frequency doubled by SHG to 308 nm. The nonlinear conversion takes place inside a BBO crystal, which is placed within an optical enhancement cavity (Sirah GmbH WaveTrain 2). The enhancement cavity is stabilized with the Pound-Drever-Hall (PDH) technique in order to track changes of the frequency of the input wave at 616 nm[196]. The beat note at 616 nm is used in a PLL which controls the fast and the slow PZTs of the dye laser cavity (Section 3.2.4).
Before purchasing this dye laser system, we tried to construct a 308-nm spectroscopy laser based on a diode laser. The following sections detail the results of these efforts.
9.4. UV Spectroscopy Laser System 9.4.1 Laser Diode 1st Setup
There are no commercially-available laser diodes that emit in the wavelength region around 308 nm. Despite this, two laser diode based techniques for generating light around 308 nm for OH spectroscopy have been implemented. The first technique involves cooling a laser diode specified for operation at 635 nm to a low temperature. The band gap of the semiconductor increases at lower temperatures, resulting in emission at a shorter wavelength[197]. The target wavelength is 616 nm, which is afterwards frequency doubled to 308 nm. The second technique involves the sum frequency generation of a violet diode laser at 404 nm and a distributed feedback (DFB) laser diode at 1320 nm inside a BBO crystal[92]. The generated wave around 308 nm is tuned continuously by ramping the current or the temperature of the DFB laser diode. However, both laser diodes require sufficient stability for precision spectroscopy.
In preliminary tests, this thesis followed the approach of cooling a single mode laser diode (Oclaro HL63163DG). The output wavelength at room temperature is 633 nm with a maximum output power of 100 mW with a current of 230 mA. The recommended operating temperature range is between −10◦C and +40◦C, but this is violated with the following setup. The laser diode is mounted inside a copper block, which is attached through a polytetrafluoroethylene (PTFE) insulator at the bottom of a liquid nitrogen container (Figure 9.8a). An additional resistor at the copper mount is used for temperature control of the laser diode. Evacuating the region around the laser diode thermally insulates the liquid nitrogen container, which is resting on three glass spheres at the bottom, and prevents condensation on the cooled components. The laser light is collimated with an aspherical lens and coupled out through a Brewster window. The temperature dependent wavelength shows approximately a linear trend (Figure 9.8b). However, the decreasing
(a) (b)
Figure 9.8: (a) Dewar vessel for liquid nitrogen with a laser diode placed inside the evacuated space. (b) The laser diode emission wavelength decreases with temperature along with its ability to operate on a single mode.
temperature moves the laser diode operation further away from its design temperature.
The laser diode becomes less stable with decreasing temperature, and it operates on multiple longitudinal modes at low temperatures. The wavelength meter (HighFinesse
Chapter 9. Experiment
WS7) tends to misinterpret the wavelength during multimode operation, which results in a large spread of the wavelength readings at low temperatures. The general trend is highlighted with a 2nd-degree polynomial fit (Figure 9.8b). However, even without single mode emission, this setup demonstrates the emission of 616 nm light with an intensity of around 200 mW (200 mA) at a temperature of−130◦C. Encouraged by this measurement, we constructed a second setup with the goal of increasing the stability of the laser diode.
9.4.2 Laser Diode 2nd Setup
The previous laser diode setup displayed poor wavelength stability along with insufficient liquid nitrogen reservoir size requiring a refill approximately once each hour. Increasing the size of the dewar vessel addresses both issues (Figure 9.9a). The larger volume of
(a)
1.6 2.4 3.2
Frequency / GHz 0
1 2 3
Amplitude/a.u.
Single Mode Multi Mode
(b)
Figure 9.9:(a) Dewar vessel for liquid nitrogen with an ECDL placed inside the evacuated space. The vacuum is maintained with the help of an ion pump. (b) Successful measurement of single mode emission behind a scanning Fabry-Perot interferometer, although at most ECDL settings, the emitted light remains multi mode.
the nitrogen reservoir allows operation of the laser diode for approximately 7 h without refill. Additionally, the area below the reservoir provides enough space to implement an external cavity diode laser (ECDL)[198–201]. A grating is placed in front of the laser diode in Littrow configuration, meaning the first order diffracted beam is coupled back into the diode. The efficiency of the holographic grating (Throlabs GH13-24V) is approximately 17 % for parallel polarized light at 616 nm. The coarse alignment of the grating requires two rotational feedthroughs into the vacuum for the vertical and the horizontal tilt of the grating (Figure 9.9a). An additional stacked PZT (Throlabs PK4DMP2) allows fine-tuning of the horizontal tilt with a travel range of 9.2µm. All electrical components inside the evacuated region of the dewar vessel are connected through vacuum-compatible Kapton-insulated wires. The additional ion pump improves the vacuum inside the dewar vessel.
For the following measurements, the laser diode is operated at 200 mA and cooled down to−137◦C. The laser beam is coupled into a scanning Fabry-Perot interferometer (FPI) (FSR=1.5 GHZ). At single mode emission, two distinct interference peaks separated by the
free spectral range (FSR) are detected with a photodiode behind the FPI (Figure 9.9a).
9.5. IR Spectroscopy Laser System