For certain atomic processes, we require that the frequency spread of our lasers be smaller than the atomic linewidth. Because of this, a free-running diode is not sufficient. To solve this problem, we turn to an external-cavity for additional frequency stabilization.
The external cavity diode laser (ECDL) was designed to reduce the linewidth of a typical diode laser. It operates under the principle that if we inject light of a very specific wavelength back into the diode, the diode will lock on to this mode and oscillate only with that specific color. This is accomplished by using a grating and a mirror, such that the grating angle will change the color of light reflected back into the laser diode. There are two common optical setups used to accomplish this; The Littrow configuration [47–49] involves reflecting the -1 grating order directly back into the diode [See Fig. 4.2 (left)] and the Littman-Metcalf [50, 51] method involves using an additional mirror to stabilize the output beam [See Fig. 4.2 (right)]. By including the external cavity, we have added the diode angle as a method of laser-frequency manipulation. This method will reduce the linewidth of the free-running diode by many orders of magnitude, more than enough to address specific atomic hyperfine transitions. However, we must now stabilize the lasers to prevent frequency drift.
By nature, an ECDL is sensitive to many perturbations in its environment. The main quantities of importance here are the laser-diode current, the laser-diode temperature, and the angle of the cavity grating. Depending on the length and construction of the cavity, the angle of the cavity may also become dependent on the temperature of the entire laser. This means that the laser requires active current and temperature stabilization. A constant current can be ensured by using a low-
4.1. LASER SYSTEMS 51
Figure 4.2: Two different methods for narrowing a diode laser’s linewidth via external cavity are shown. The Littrow (left) configuration feeds the -1 order from the diffraction grating back into the laser diode to create a cavity (dashed line). The output beam scans as the angle of the grating is scanned. The Littman-Metcalf method (right) adds a mirror, which makes a longer external cavity (dashed). This method has a lower coupling back into the diode, and has an output which is insensitive to frequency scanning. A steering mirror can be added to a Littrow configuration to make output alignment insensitive to laser frequency.
noise power supply for the laser-diode. A constant temperature must be maintained to sub-degree precision, which can be accomplished using electronic coolers in direct thermal contact with the laser-diode head.
In addition to these stringent requirements on laser parameters, we also require that back-scattered light not be introduced into the diode. Since the cavity is extremely important to the single-mode operation of the laser, additional light from another source would be bad for the stable operation of the diode. Because of this,
we implement an optical isolator after every ECDL used in our experiment. An optical isolator consists of two polarizers rotated by π/4 from one another and a highly magnetic polarization-rotation element (Faraday rotator) in the center. If we match the output polarization with the input polarizer of the isolator, the light enters the Faraday rotator and is rotated to match the polarization of the output. However, backwards-traveling light will enter the output of the isolator and rotate
−π/4, making it completely orthogonal to the input polarizer. As a result, backwards-
propagating light is absorbed by the isolator and does not reach the laser-diode. In addition to wavelength spread, another important quality of an ECDL is mode- hop free tuning. A mode-hop occurs when the laser preferentially chooses a different mode to lase on, which is typically a few hundredths of a nm away from the previous mode. This is all determined by the mode spacing of the laser. This can be a problem when scanning the laser, or looking for a resonance. All of our lasers implement a feed-forward, which feeds a voltage proportional to the laser-current to the piezo controlling the grating angle. This will match the cavity conditions over a larger range of tuning, and can drastically improve the mode-hop free scan range.
In the experiments here we require the atomic transitions of both Na and Ca to be addressed, as well as those of ionized Ca+. Because we would like to address individual states with accuracy, we require an ECDL. The neutral Ca is pumped with a commercial 423 nm ECDL as the first step to the ionization process. The 375 nm laser mentioned earlier ionizes this excited Ca. In order to address the individual states of the Ca+ ion, we require 397 nm and 866 nm radiation to pump on the
S → P and D → P transitions respectively. All of the lasers mentioned here
are Toptica diode lasers. The 866 nm laser is a new Toptica DL Pro, which has increased stability and power output. Previous incarnations of the 866 nm laser were
4.1. LASER SYSTEMS 53 homebuilt, and were very susceptible to environmental factors like construction noise and temperature variability. Unfortunately, it could not be stabilized using the above mentioned methods1.