Reading time: 1 min 42 sec

It is quite possible to view the pulse stream of a femtosecond output on an oscilloscope screen, especially to measure the repetition rate of the laser. It is to be noted that, we cannot see the pulse shape of the laser beam as the response time (rise and fall time) of the photodiode (PD) is quite higher than the pulse width of the femtosecond laser.

Things to be noted:

1. The damage threshold of the PD: Since many optics companies have not measured the damage threshold of PDs with a femtosecond laser, it is advisable to reduce the power of the laser before it hits the PD. TO do this, PD can be set in front of the reflected light from a piece of glass plate so that only 4% reflected power is incident on the PD.
2. Parameters of the oscilloscope: Oscilloscope should have a bandwidth of at least three or four times higher than the expected repetition rate. So, the oscilloscope can measure the frequency value with better accuracy.
3. The sampling rate of the oscilloscope: Better the sampling rate, better the signal resolution along the x-axis of your oscilloscope. The sampling rate can be changed by tuning time/div on the oscilloscope as the memory depth of the oscilloscope is constant. Smaller the time/div higher the sampling rate. 10 ns/div is preferred so that a sampling rate of 10 GS/s is achieved in GW Instek GDS 3504 oscilloscope.
4. Terminal impedance: When a PD is connected to the oscilloscope through a BNC cable, the input impedance of the oscilloscope (terminal impedance of the BNC cable) should be very small. Most oscilloscopes have a default value of 1 MOhms. This is to provide a better output voltage on the oscilloscope and works fine when the input frequency is low. But if the repetition rate (frequency of the circuit) is in the order of MHz, we should go for low impedance (50 Ohms), so that frequency response is well monitored by the oscilloscope.
5. Response time, wavelength range and bandwidth of PD: Response time of PD is a crucial factor. Rise time in the order of ns is suitable. Also, make sure that the wavelength range and bandwidth of the PD are ideal for this application. Thorlabs DET10A PD is recommended as it has a rise time of 1 ns@ 632 nm and bandwidth of 350 MHz for a femtosecond laser  (pulse width: 160 fs, Rep. rate: 76 MHz. Please see the website of Thorlabs for more details about the detector).

Pulse stream on the oscilloscope:

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