Narrow-linewidth and frequency stable lasers are nowadays a key components in order to reach high performances in many domains and applications where laser interferometry process is involved, like for LiDAR, SENSING, METROLOGY, QUANTUM TECHNOLOGIES, … Making a narrow linewidth laser that is compact, low cost and can handle hard environment (temperature, acoustics, vibrations…) is very challenging but not impossible.
The best way to characterize and stabilize the laser is by being able to detect the laser frequency fluctuations, which is done with an OPTICAL FREQUENCY DISCRIMINATION scheme. At SILENTSYS, we propose plug and play OPTICAL FREQUENCY DISCRIMINATOR (OFD) in order to reach high performances laser frequency stabilization / characterization.
We will see here what are the main characteristics of an OFD and how to choose them properly demanding on different parameters.
OPTICAL FREQUENCY DISCRIMINATION : DEFINITION
OPTICAL FREQUENCY DISCRIMINATION, or LASER FREQUENCY DISCRIMINATION is a technic that gives the frequency evolution in time domain of a single frequency under-test laser as we have presented in another article. So, these are methods that convert laser frequency emission into a readable signal, because, in contrast to electronic oscillators, a laser emits an electric field at very high frequency typically, around 200 THz for telecom lasers (1,5µm wavelength).
At SILENTSYS, we are also doing OPTICAL FREQUENCY DISCRIMINATION, that is directly integrated in few of our products like the OFD (Optical Frequency Discriminator) or the OFC (Optical Frequency Correlator). We provide OPTICAL FREQUENCY DISCRIMINATION that are small, fast, affordable and available from 400nm to 2200nm with a typical working range of 100nm around the central wavelength.
These are based on modified Michelson or Mach-Zehnder fiber interferometers, with a high vibrations / acoustics isolation and high-level temperature stabilization (at the micro-Kelvin level) and ultralow noise photo-detection in order to reach very good laser frequency discrimination precision.
These products make good candidates for laser frequency stabilization or characterization, enabling Frequency Noise PSD measurement, Frequency Stability analysis, and Frequency Noise reduction by more than 60dB to reach Hz-level laser linewidth from a MHz one. The frequency stabilization is very easy as there is no need for external laser frequency modulator, complex locking scheme (as Pound-Drever-Hall), and no need for cavity scanning as with OFD/OFC there is a locking point each half of FSR, so for example each 5MHz (FSR of 10MHz) compared to each 1GHz with a 15cm-long FP cavity.
HOW DOES IT WORK ?
As the SILENTSYS OFD/OFC are based on a modified Michelson or Mach-Zehnder interferometers, so a 2-waves interferometric process, the result is a sine fringes pattern by the change of the input laser frequency. At the middle of one fringe (the green zone on the Fig.3), the conversion between laser frequency fluctuations and output voltage is linear, so it is then possible to retrieve the laser frequency fluctuations by recording the voltage time trace of the OFD and by knowing 2 important parameters which are: the amplitude of the fringes (Vmax) and the OFD free spectral range (so the frequency distance between 2 fringes).
In the next figure, as the laser is going through 4 fringes by changing its frequency of 4 MHz, it means that the FSR is 1 MHz in this specific case. The OFD FSR is fixed by construction and can be adjusting at the order between 2MHz and 2GHz (other values possible on demand).
HOW TO PROPERLY CHOOSE THE FSR VALUE?
As explained just above, the FSR (Free-Spactral-Range) is an essential parameter and fixed at the device manufacturing. The FSR defines both the frequency to voltage sensitivity (lower is the FSR and higher is the sensitivity in V/Hz) and the detection bandwidth.
So, lower is the FSR and better it is to be able to measure / stabilize with lower frequency noise floor, but in another hand, lower is the FSR and lower is the measurement / stabilization bandwidth. The -3dB bandwidth is approx. the FSR divided by 2.
To summaries, it is important for laser frequency stabilization / characterization to be well around the quadrature point, meaning here around 0V. This is here that the conversion is linear. This makes that the laser frequency excursion should be less than approx. FSR/4 to stay in the linear range and have good frequency stabilization / characterization. This avoid non-linearities during the conversion.
Saying it, it implies a huge constraints regarding the laser frequency noise (linewidth) when free-running to the OFD-FSR.
- High laser frequency noise -> High FSR mandatory
- Low laser frequency noise -> Low FSR possible
Usually, the internal rule we have is to set the OFD-FSR minimum 100 times higher than the white frequency noise of the laser (for laser frequency stabilization). For example, for a laser of an instantaneous linewidth of 300kHz (so a white frequency noise floor of 105 Hz²/Hz) we recommend to not use an FSR below 10MHz.
The next figure shows the impact of the white laser frequency noise on the out signal of the OFD for a given FSR. As we can see, while the laser is locked (constantly around 0V), the frequency noise at high frequencies is not corrected because of being out of the locking bandwidth. If the FSR is too low compared to the thickness of the trace (i,e the fast laser frequency excursions) this can cause laser frequency jumps (OFD mode-hopes) and also increasing of the frequency noise floor due to non-linearities.
Moreover, if the goal is to evaluate the frequency drift or the frequency noise of a laser that is moving highly at “low” frequencies like at the ms range for example, it is important that the laser stay inside the linear conversion range, so again that the excursion during the measurement is low enough. The next figure shows the result of a frequency modulated laser and a non-modulated laser. Actually, the modulated laser can just be an ECDL that is very sensitive to the vibration and placed in a wrong environment !
SUMMARY
To conclude this article, here are the typical rules we suggest to correctly choose the OFD FSR (few of them need compromises):
- FSR about 100 times the laser white frequency noise (recommended especially for locking)
- Frequency noise measurement / locking bandwidth is typically FSR divided by min 2 (and FSR divided by 4 for locking)
- FN-PSD measurement noise floor is improved with lower FSR. Lower sensitive, relatively, to photodetection noise floor for example.
- Laser frequency excursions of typ. FSR divided by 4 during the measurement (to stay in the linear conversion range). We recommend even a factor 10.
