A semiconductor laser emits light by a driving current. This drive current causes the semiconductor laser chip to generate heat, causing a shift in the output wavelength and potentially damaging the laser. To avoid such problems, pulse-driven light emission is widely used. Furthermore, high-power industrial lasers may require an energy accumulation period before the power is released as a short pulse. The pulse drive conditions of these lasers are designed with repetition frequencies and duty ratios that depend on the application. To accurately measure pulsed light using an optical spectrum analyzer (OSA), it is necessary to understand the characteristics of the OSA and select the appropriate measurement method and settings.
An OSA samples and measures the total power of a specific wavelength segment while sweeping the wavelength and displays it as an optical spectrum. Normally, it is assumed that the optical input signal is constant during the OSA wavelength sweep. In other words, a signal that turns the optical input signal on or off, such as pulsed light, is not expected. Therefore, if the OSA measurement method and condition settings are not suitable for the pulsed light drive conditions, the measured spectrum may appear segmented or choppy, as shown in Figure 1.
Figure 1. A pulsed input causes a segmented or choppy trace.Depending on the driving conditions of the pulsed light, Yokogawa Test&Measurement OSAs offer three types of measurement methods: time average spectrum measurement, external trigger synchronous measurement, and peak hold measurement.
In time average mode, the time average power of pulsed light is measured as the optical spectral power of each wavelength. When the pulsed light is a square wave, the measured average light power is calculated as:
(Peak Power of Light Pulse [mW]) × (Duty Ratio of Pulsed Light)
Therefore, the smaller the duty ratio of pulsed light, the lower the measured power. To measure the average power correctly in this mode, the repetition frequency must be high to some extent. Otherwise, it may need to be measured in a high-sensitivity setting or by increasing the number of averages.
The pulse repetition frequency is roughly divided into the following three groups, and the measurement method can be considered accordingly.
The table in Figure 3 shows the estimated minimum repeat frequency that can be measured with each sensitivity setting.
Figure 3. Estimated minimum repetition frequency. HIGH1-3 of AQ6375/76/77 is in high dynamic range mode (CHOP) and the averaging does not reduce the minimum repeat frequency in this mode.The time average mode can measure high-power pulsed light with a peak power of more than 1W. However, pulsed light of which average spectral power exceeds the OSA maximum input power specification cannot be measured.
[Caution] There is a limit to the optical power that can be input to the OSA. See “Guidelines for Pulsed Light Input Power” in this app note for more information.
Figure 4. Maximum input power is the maximum spectral power per measurement resolution.The external trigger mode uses an external electrical trigger signal to control the timing of OSA measurements. Data measurement, or signal sweeping, starts when triggered by an external electrical trigger to the input terminal on the rear panel of the instrument. Supplying an external trigger signal synchronized with the optical pulse signal to the OSA captures the peak value of the optical pulse. The OSA takes one sample of the detection circuit each time it receives the trigger signal from the laser pulse, then increments it to the next wavelength step to take the next sample when it receives the next trigger pulse signal. Refer to the user’s manual of a model to view their specific electrical properties of trigger signal.
The external trigger mode requires a pulse width of at least 50 μs to capture the peak of the optical pulse. The minimum pulse width depends on the measurement sensitivity setting used, and the higher the measurement sensitivity, the wider the minimum pulse width at which peaks are captured.
The suitability of this mode is mainly determined by the pulse width to be measured because the gain of the detection circuit and the response speed are inversely proportional. At low sensitivity settings, the detection circuit gain is low and the response is fast, allowing for the correct measurement of short pulse width peaks. At high sensitivity settings, the gain of the detection circuit is high and the response is slow, so short pulse width peaks cannot be measured correctly.
The table in Figure 5 shows the estimated minimum pulse width that can be measured with each sensitivity setting.
Figure 5. Estimated minimum pulse width. HIGH1-3 of AQ6375/76/77 is in high dynamic range mode (CHOP) and the AQ6360 does not have the external trigger mode.To measure the peak power of the optical pulse, connect an external trigger signal synchronized with the optical pulse to the TRIGGER IN port on the back of the OSA, and set TRIG INPUT MODE in the OSA system menu to SMPL TRIG MODE. The SMPL TRIGGER MODE triggers at the rising edge (or falling edge) of the input external trigger signal to measure 1 point. Adjust the timing as needed to capture the peak of the optical pulse correctly. The OSA has a delay of about 70 μs from trigger to measurement. It also has a function to add a delay time in the range of 0 to 1000 μs.
In external trigger mode, OSA captures the peak power of the optical pulse. Optical pulsed light that exceeds the OSA maximum input power specifications cannot be measured. The peak power of a measurable optical pulse is at most +20 dBm (0.1 W). Figure 6 shows the maximum input power of each model.
[Caution] There is a limit to the optical power that can be input to the OSA. See “Guidelines for Pulsed Light Input Power” in this app note for more information.
Figure 6. Maximum input power is the maximum spectral power per measurement resolution.Peak hold mode is a measurement mode that does not require an external trigger signal. The detection signal is recorded for the specified period for each measurement wavelength, and the maximum value of the data acquired during that period is used as the power for that measurement wavelength. The period during which the detection signal is recorded is called the "hold time" and is set to a value larger than the pulse repetition period (1/repetition frequency). This allows users to measure at least one pulse within the hold time. The OSA essentially takes one sample from one laser pulse and repeats the movement of the measurement wavelength and the recording of the output signal of the detection circuit.
The peak hold mode requires a pulse width of at least 100 μs to capture the peak of the optical pulse. In addition, the minimum pulse width depends on the measurement sensitivity setting used; the higher the measurement sensitivity, the wider the minimum pulse width at which peaks are captured. The suitability of this mode is mainly determined by the pulse width to be measured because the gain of the detection circuit and the response speed are inversely proportional.
At low sensitivity settings, the detection circuit gain is low and the response is fast, allowing for the correct measurement of short pulse width peaks. At high sensitivity settings, the gain of the detection circuit is high and the response is slow, resulting in incorrect measurements of short pulse width peaks.
The table in Figure 7 shows the minimum pulse width that can be measured with each sensitivity setting.
Figure 7. Estimated minimum pulse width. HIGH1-3 of AQ6375/76/77 is in high dynamic range mode (CHOP) and the AQ6360 does not have the external trigger mode.Set the hold time to a value larger than the pulse repetition period (1/repetition frequency). With an OSA, the hold time can be set arbitrarily within the range of 0 to 9999 ms and it can measure down to a pulse repetition frequency of 0.1 Hz.
In peak hold mode, since an OSA captures the peak power of the optical pulse, an optical pulsed light that exceeds the OSA maximum input power specifications cannot be measured. This results in the peak power of a measurable optical pulse being at most +20 dBm (0.1 W). The maximum input power of each model is shown in Figure 8.
[Caution] There is a limit to the optical power that can be input to the OSA. See “Guidelines for Pulsed Light Input Power” in this app note for more information.
Figure 8. Maximum input power is the maximum spectral power per measurement resolution.When measuring pulsed light, it is recommended to use the pulse peak power below the maximum safe input power of the OSA to prevent damage inside the equipment. Since the limitation differs depending on the emission condition of the pulsed light, there are some additional considerations, listed below. It should be noted that these are merely estimated values and do not guarantee safety.
When the pulse width exceeds 1 µs
When the pulse width is 1 µs or less
For pulsed light measurements, it is best to first ensure compliance with the recommendations in “Guidelines for Pulsed Light Input Power.” The time average mode is the easiest and most flexible way to measure pulsed light.
In some instances, though, it may make better sense to consider using the external trigger or peak hold modes. Examples include when irregular pulsed light is present, when measurement is not possible because the average power is below the measurement sensitivity due to the relationship between the peak power and the duty ratio, and when measurement takes too long because the high sensitivity setting is required due to very low repetition frequency.
The suitability of using either the external trigger mode or peak mode is determined by the pulse width and the required sensitivity setting, or by the availability of an external trigger and the minimum pulse width.
Figure 11. OSA setting guide for pulsed light measurement.Any measurement mode can be selected according to the pulse width condition. If there is an external trigger signal, select "external trigger." Otherwise, select "peak hold." For external trigger and peak hold, make sure that the pulse peak is below “maximum input power."
Figure 12. Pulse measurement modes for 100 µs pulse width."Time average" or "external trigger" can be selected according to the pulse width condition. Select "external” if there is an external trigger signal, as this is faster. In the case of "external trigger," make sure that the pulse peak is below the maximum input power. In the case of "time average," make sure that the pulse peak is below the maximum safe input power according to the pulse width condition, and set the sensitivity setting to HIGH2 or higher according to the repetition frequency.
Figure 13. Pulse measurement modes for 50 µs pulse width.Select "time average" according to the pulse width condition. Make sure that the pulse peak is below the maximum safe input power according to the pulse width condition. Set the sensitivity setting to HIGH2 or higher according to the repetition frequency.
Figure 14. Pulse measurement mode for 1 µs pulse width.Select "time average" according to the pulse width condition. Make sure that the pulse peak is 316 W or less and the average power is below the maximum safe input power according to the pulse width condition. Set the sensitivity setting to HIGH2 or higher according to the repetition frequency.
Figure 15. Pulse measurement mode for 1 ns pulse width.The OSA sensitivity setting has a significant effect on the quality and duration of pulsed light measurements. In time average mode, the sensitivity setting limits the measurable pulse repetition frequency, and in external trigger mode and peak hold mode, the minimum pulse width is limited. These are related to the response speed of the OSA detection circuit.
The measurement sensitivity setting determines which OSA detection circuit is selected. Each circuit corresponds to each sensitivity setting and provides a specific gain and response speed. The gain and response speeds are inversely related – increasing the gain slows down the response speed. At high sensitivity settings such as HIGH1, while the gain of the detection circuit increases, both the reaction speed and wavelength sweep become slower. Conversely, with low sensitivity settings such as Norm/Hold and Normal, the gain of the detection circuit is reduced, while the reaction speed and wavelength sweep are faster.
Figure 16. Sensitivity settings influence response time.In time averaging mode, if the OSA responds too quickly compared to the pulse repetition frequency, there will be a clear on/off distortion in the recorded spectrum. Therefore, it is very important to select the appropriate measurement sensitivity for the pulse repetition frequency.
In external trigger mode and peak hold mode, peaks of short pulse width can be measured correctly at low sensitivity settings where the gain of the detection circuit is low and the response is fast. However, at high sensitivity settings, the gain of the detection circuit is high and the response is slow, resulting in incorrectly measured short pulse width peaks. Therefore, it is very important to select the appropriate measurement sensitivity for the pulse width to be measured.
In the low sensitivity setting, P(λ) is high because the response of the detection circuit is fast. The peak of the pulse is captured. At each wavelength, the pulse detector signal is recorded at specific time intervals. The maximum signal acquired at the time interval is used as the power value at a specific wavelength. In the high sensitivity setting, P(λ) is low because the response of the detection circuit is slow. The peak of the pulse cannot be captured.
Figure 17. Laser output pulse signal. Figure 18. Detection circuit output signal (low sensitivity). Figure 19. Detection circuit output signal (high sensitivity). Figure 20. Operation in peak hold mode.
High Performance LONG WAVELENGTH
The AQ6375B is a bench-top optical spectrum analyzer covering the long wavelengths, 1200 to 2400 nm, with the added benefits of gas purging input ports / output ports, a built-in cut filter for high order diffracted light, and a novel double speed mode which increases the sweep speed up to 2 times compared to the standard sweep mode.
High Performance LONG WAVELENGTH
The AQ6375E covers not only telecommunication wavelengths, but also the SWIR region which is often used for environmental sensing and medical applications.
・Lineup of 3 models [Standard, Extended and Limited]
・Covers wavelengths
1200 to 2400 nm [Standard, and Limited]
1000 to 2500 nm [Extended ]
MWIR WAVELENGTH with internal gas purge and cut filter
The AQ6376 is the latest version of our bench-top optical spectrum analyzer extending the wavelength coverage well beyond the NIR range of our previous models into the MWIR region from 1500 to 3400 nm.
Popular applications include the detection of gases such as carbon oxides (COx), nitrogen oxides (NOx), and hydrocarbon gas (CxHy) for environmental studies.
MWIR WAVELENGTH with internal gas purge and cut filter
The AQ6376E is the latest version of our bench-top optical spectrum analyzer extending the wavelength coverage well beyond the NIR range of our previous models into the MWIR region from 1500 to 3400 nm.
Popular applications include the detection of gases such as carbon oxides (COx), nitrogen oxides (NOx), and hydrocarbon gas (CxHy) for environmental studies.
AQ6380 OSA: 5 pm (0.624 GHz) high wavelength resolution, ±5 pm accuracy, 65 dB wide close-in dynamic range, 80 dB high stray light suppression
Unique LONG wavelength range of 1200 to 2400 nm makes this the world's first and only OSA specifically designed for use in advance applications such as Carbon Monoxide (CO), Carbon Dioxide (CO2) gas detection and LIDAR.
Measures the power intensity of light across different wavelengths in the electromagnetic spectrum.