Let’s talk about Keysight’s tunable laser source
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Let’s talk about Keysight’s tunable laser source

Posted Date: 2024-01-25

Author: Gu Lei

In 1970, Corning of the United States made a breakthrough by turning the high-K fiber theory into reality and successfully prepared the world's first low-loss optical fiber, marking the beginning of the era of optical fiber communications. With the rapid development of optical fiber communications, the wavelength, linewidth, and adjustment interval of tunable lasers have been greatly improved, and thus they are widely used in the fields of optical fiber sensing and photoelectric measurement.So in the first article about silicon light, let’s talk about Keysight’s tunable laser source.

Keysight's tunable laser sources adopt an external cavity tunable laser structure and are divided into two types: modular (discontinued) and independent laser sources. It can provide a highly stable and highly reliable tunable narrow-band light source for wavelength-related testing of optical passive devices. It is mainly used as a local oscillator source for coherent optical communications or used with optical power meters, polarization synthesizers and other equipment for high-speed, high-resolution wavelength-related parameter testing of passive optical components and materials.

N777XC series

index N7776C N7778C N7779C Wavelength range #013: 1250~1370nm(19dBm)
#113: 1240-1380nm
#114: 1340-1495nm
#116: 1490~1640nm
#216: 1450~1650nm #013: 1250~1370nm(19dBm)
#113: 1240-1380nm
#114: 1340-1495nm
#116: 1490~1640nm
#216: 1450~1650nm #113: 1240-1380nm
#114: 1340-1495nm
#116: 1490~1640nm
#216: 1450~1650nm Maximum scanning speed 200 nm/s Bidirectional 200 nm/s Bidirectional no maximum peak power >12 dBm >12 dBm >12 dBm Static wavelength absolute accuracy ≤±1.5 pm typ. ≤±4 pm typ. ≤± 10 pm typ. Absolute accuracy of swept wavelength ≤±1.5 pm typ. ≤±4 pm typ. Relative accuracy of non-static wavelength ≤±1.0 pm typ. ≤±3 pm typ. ≤±5 pm typ. Relative accuracy of swept wavelength≤ ±1.0 pm typ. ≤±2.5 pm typ. No static wavelength repeatability ≤±0.2 pm typ. ≤±1 pm typ. ≤±3 pm typ. Sweep wavelength repeatability ≤±0.3 pm typ. ≤±1 pm typ. . No 24-hour wavelength stability ≤±0.5 pm typ. ≤±1.5 pm typ. ≤±5 pm typ. 24-hour power stability ≤±0.025 dB typ. ≤±0.03 dB typ. ≤±0.03 dB typ. Power repeatability Characteristic ≤±0.002 dB typ. ≤±0.01 dB ≤±0.01 dB Power linearity ≤±0.05 dB ≤±0.1 dB ≤±0.1 dB Signal ratio total SSE ≥75 dB ≥70 dB ≥70 dB Side mode suppression ratio ≥70 dB typ. ≥70 dB typ. ≥70 dB typ. Relative intensity noise line width minimum wavelength resolution 0.1 pm 0.1 pm 0.1 pm

then, let's watch it togetherThe main uses of tunable laser sources:

1

Insertion loss test (IL)

A swept insertion loss test system composed of a tunable light source combined with one or more optical power meters can be used to test optical power vs. optical wavelength. This test system is usually used to test the ratio of input optical power to output optical power of optical components. This ratio is the insertion loss, usually measured in dB. When the tunable laser (TLS) performs wavelength scanning at a fixed step in a fixed wavelength range, the optical power meter (PM) periodically samples the optical power point. The optical power meter and the tunable light source are synchronized through a trigger signal, so the power of the sampling point recorded by the optical power meter corresponds to the wavelength of the tunable light source. The multi-channel power meter allows several channels to simultaneously collect optical power points, so it is very suitable for testing multi-channel optical devices such as splitters, Mux, wavelength switches and other devices. The tunable light source of this test system can be selected from N7776C and N7778C of the N777xC series. The optical power meter can be selected from the N774xC series of multi-channel optical power meters and used with Keysight's free N7700A IL Engine software to implement sweep testing.

2

Combined with optical switches to achieve full-band insertion loss testing

The wavelength coverage of three tunable light sources can be from O band to L band. Together with one N7731C optical switch and multi-channel optical power meter, optical power vs optical wavelength testing covering the entire O band to L band can be achieved.

3

Polarization Dependent Loss (PDL)

The insertion loss of optical devices is often related to the polarization state and degree of polarization of the incoming light. Therefore, in order to accurately know the insertion loss of the device, it is necessary to determine the polarization state of the light. The polarization-dependent loss test involves determining the maximum and minimum insertion loss of the optical device at each wavelength point after traversing the polarization states. However, the insertion loss test that traverses the polarization states at each wavelength point is extremely time-consuming. Keysight uses the 4-state or 6-state Mueller matrix method to test the polarization-related loss at each wavelength point, which greatly reduces the test time. Compared with the pure insertion loss test system, this system adds an N7786C polarization analyzer after TLS to control the polarization state of the light entering the DUT. The new N7786C polarization analyzer has extremely fast polarization switching capabilities and can monitor the state of polarization (SOP) and power of light in real time, so the entire Band PDL test only requires one TLS scan to complete. This system, coupled with the software N7700100C (Lambda Scan), can not only complete high-speed PDL testing, but also complete testing items of polarization-related parameters such as TE/TM spectral shape.

4

Optoelectronic device wavelength polarization related testing

More and more optical modules integrate photodiodes (PDs) with passive optical devices and circuits. for example:

a) Integrated coherent receiver (ICR)

b) ROSA device

c) Optical channel monitor (OCM)

These devices under test all have optical input ports, electrical or RF output ports. Before entering the photodiode, the light will first pass through passive optical devices such as polarizers, beam splitters or interferometers, and then the photodiode will convert the optical signal into a photocurrent signal. Therefore, the unit of the photodiode responsivity parameter is mA/mW, which is also affected by the wavelength and polarization of the light. The test of this system is similar to the previous PDL test plan. The only difference is that the optical power meter is replaced by a source meter to periodically sample and record the electrical signal. This test system requires the support of software N7700100C (Lambda Scan).

By comparing the input optical power and output photocurrent of the sweep test, the responsivity/response curve of the device is the average value of the responsivity in each polarization state. At the same time, the maximum responsivity and minimum responsivity curves (TE/TM curves) of the device in different polarization states will also be obtained, which is extremely useful for testing polarization-related devices (such as ICR). The common mode rejection ratio parameters of balanced devices can also be measured under this system.

For testing similar ICR devices, it is often necessary to apply a DC bias to the pins of the device, and the photocurrent is then converted into an RF signal for output. In order to have higher flexibility on the combiner, it is recommended to use the B2900A series source meter accessories.

5

Polarization mode dispersion PMD, differential group delay DGD test

Keysight pushes the limits of optical component measurement with the N7788C. For device PMD or DGD testing, the test results of N7788C's unique patented test method completely match the results measured by the traditional standard Jones Matrix Analysis Method (JME). Compared with JME method testing, Keysight's N7788C can not only test the PMD and DCD parameters of the device in a single scan, but also obtain more complete optical parameters of the device as follows:

• Differential group delay/polarization mode dispersion/polarization dependent loss/2nd order polarization mode dispersion

• Power/insertion loss

• TE / TM losses

• Principal states of polarization (PSPs)

• Jones and Muller matrices

In order to test all the above parameters, the hardware requires the N7788C to be used with a tunable sweep light source such as N7776C or N7778C, and the software requires the support of the N7700103C test software.

The above is an introduction to Keysight’s tunable laser sources and their applications. It should be noted that as of December 2023, the modular laser source host 8164B and laser source modules (81950A/81600B, 81602A/81606A, 81607A, 81608A, 81609A) have been discontinued. The new generation N777XC series laser source will better support related tests.

Review Editor: Huang Fei


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