3.1

Optical pulse generator system

The optical pulse generation system is formed by arbitrary waveform generator (AWG), high bandwidth linear current driver, efficient TEC controller and an integrated laser diode (LD). Due to the required pulse width of the short pulse, a wave generator with a minimum clock of 0.1 GHz is necessary. For this purpose, the OPM-LD-ps module from the manufacturer Optical pulse machines (OPM) was acquired to generate both the short and long pulse. This module is a “sandwich” of two boards: the arbitrary wave generator and a high current (HC) electrical-to-optical (EO) converter board (model OPM-LD-Lin-HC). The AWG’s clock rate is 1.8 GHz which enables generation of waveforms in subnanosecond resolution. The HC-EO converter board allows a laser peak current up to 2 A. This emulator works with a laser diode (LD-1064-PM-p1200, Innolume). The mean wavelength is 1064.372 nm and FWHM is 3.264 nm (conditions: 1% duty cycle, 500 ns pulse width, pulse operation). The peak power is 1.2 W @ 2 A and threshold current is 61 mA. The type of optical fiber is PANDA PM980 (the output light is polarized along the slow axis of PM fiber), the core diameter is 5.5 μm and the mode-field diameter (MFD@1064 nm) is 7.2 μm.

Figure 2.

Block diagram of the complete system.

The maximum number of samples to build the waveform is 55,200. Since the AWG is clocked at 1.8 GHz, each sample corresponds to 0.5556 ns. Therefore, a short pulse of 10 ns consists of 18 samples. The long echo with the pulse width of 270 μs cannot be generated directly from one set of samples due to its maximum length of 30.669 μs. Fortunately, the OPM-LD-ps provide six multipliers (x1, x2, x4, x8, x16 and x32) which can be used to extend the pulse duration. For 270 μs pulse width a multiplier of 16 was used. The long pulse was generated with repetition rate in the 100-500 Hz range. This means that the duty cycle must be adjustable between 2.7% to 13.5%.

A program was created using the LabVIEW environment (see Figure 3) that allowed to configure the shape of the pulses, pulse width, period, duty cycle, bias level, trigger source, laser temperature, control low-pass filter configuration, and also enabled to display the important parameters of the laser diode in a LCD screen.

Figure 3.

Frontend of the developed software to control the LEE.

The generated optical power feed either the UV generation system or the VIS – NIR stages. The optical power from the laser at 1064 nm is distributed in two channels by means of an optical coupler (TN1064R1A1B, Thorlabs). In the first channel, 99% of the optical power is used to generate the SHG in the VIS stage and both echoes. The rest of the optical power (1%) from the second channel is used to generate the short or long echo in the NIR stage. The details will be described in the following sections.

3.2

Near-infrared stage

In the second channel (after the coulpler along the 1 % path), a voltage-controlled optical attenuator (V1000A, Thorlabs) was placed. Its working principle is as simple as applying a voltage within the range from 0 to 5 V which is proportional to the attenuation from 0 to 45 dB. Variable attenuator allowed easily adjusting the power levels for both echoes between their maximum and minimum values.

To obtain the echoes, a 50:50 optical coupler (PMC-001-T-10-50-FC/APC, General Photonics) was used to divide power into two equal parts. One branch was used to generate the short echo and the other branch was used to generate the long echo. Then, a set of fixed attenuators were used to reach the peak power levels required by the project. A maximum level of 200 nW (-37 dBm) and a minimum level of 0.2 nW (-67 dBm) were required for the short echo, and for the long echo a maximum level of 25 pW (-76 dBm) and a minimum level 0.1 pW (-100 dBm). The peak power after 50:50 coupler output was meaused and depend on the obtained values, fixed attenuators were placed until the maximum level was reached. Once the maximum value was obtained, the variable attenuator was used to get any power between the maximum and the minimum level. In case of short echo branch, three fixed attenuators (two FA15T and one FA20T) were used and five (FA20T) in the long echo one. At the echo both signals can be merged by means of an additional 50:50 coupler.

3.3

Visible stage - Second Harmonic Generation (SHG)

As aforementioned, the 99% of the optical power was used to generate the SHG, which was achieved through a nonlinear crystal. The chosen crystal was a potassium titanyl phosphate (KTP). This nonlinear crystal is characterized by high nonlinear conversion coefficient, wide acceptance angle, high-allowed temperature, small walk-off angle, and relatively high damage threshold [6]. Also, it has been widely applied to generate the SHG green light, mainly, in solid state lasers (for example, neodymium-YAG laser). The dimensions of the chosen KTP are 3 x 3 x 10 mm (KTP402, Eksma Optics).

Before the light hits the crystal, a collimation stage (PAF-X-2-C-FiberPort, Thorlabs) was used to achieve a higher power at output. Once the beam at 532 nm was obtained, the residual pump light was removed through a filter (FESH0550, Thorlabs). Finally, the beam was focused into the single mode fiber with another collimator (CFS2-532-FC, Thorlabs) for further signal attenuation to reach the requirements of the project. To balance the power amount in each echo similar process as with the NIR stage was followed. A variable voltage-controlled attenuator (V450F, Thorlabs) was used to adjust the desired power once the output was calibrated to the maximum level for each echo. This attenuator has a maximum attenuation value of >30dB, the operating bandwidth between 450nm and 600nm, and the voltage operating range of 0 to 5V. Next, an optical coupler (TW560R1A1, Thorlabs) was used to divide that power into two parts (99:1). High power branch was used to generate the short echo and the low power branch was used to generate the long echo, both according to the requirements of the project. Again, some fixed attenuators were used to reach the required peak power levels in both echoes and the output can be seen in the Figure 4.

Figure 4.

Set-up of SHG (left) and the beam at 532 nm passing through the VIS collimator (right).

3.4

Ultraviolet stage - Sum Frequency Generation (SFG)

The challenge of developing this emulator was to obtain the short and long pulse at 355 nm wavelength. The SFG technique was used to obtain at 355 nm beam. Two periodically poled lithium niobate (PPLN) crystals are used because the PPLN is a highly efficient medium for nonlinear wavelength conversion processes. Usually, it is used for frequency doubling [7], difference frequency generation [8], sum frequency generation [9], optical parametric oscillation [10], and other nonlinear processes.

The Mixer module (SC19348-01-01, HC Photonics) was used to achieve the 355 nm beam at the output by introducing a 1064 nm beam as input. The input beam, which was connected to the input optical fiber (PM980) of the Mixer module, was travelling through the two PPLNs buried inside the ovens which temperatures were controlled with the TEC controllers. Temperature-tuning method was used since PPLN crystals have multiple poling period (PP) sections in each crystal, each with a different poling period, allowing different wavelengths to be used at a temperature of given crystal.

For the first PPLN, the optimal temperature was determined by adjusting the phase matching temperature of the SHG stage while controlling the optical power output with a silicon photodiode (SIL PD). The 1064nm beam was removed with a visible filter (FESH0550, Thorlabs). For this purpose, an automatic temperature scan with 0.1-degree steps was performed by a program in LabView. Once the optimum temperature of the first PPLN was reached, this value was fixed.

Afterwards, second PPLN crystal was used to generate the UV beam by applying SFG (inputs: 532 nm and 1064 nm beams). To optimize the phase matching temperature of the SFG stage, the same procedure was performed as with the SHG stage but the UV enhanced SIL PD was used in this case. Once the beam at 355 nm was obtained, it was introduced into a multimode fiber (M91L01, Thorlabs) and an UV filter (UG-11, Edmund optics) was placed before the fiber to remove the pump beams. The output beam at 355 nm measured by the CCD spectrometer can be seen in Figure 5. Due to lack of the COTS splitters with necessary features this stage is producing only long or short echo at a time.

Figure 5.

The beam at 355 nm measured by spectrometer.