A method for testing the photoresponsivity of low-frequency narrow-pulse-width AC signals

Abstract

This invention relates to a method for testing the responsivity of low-frequency narrow-pulse AC optical signals, belonging to the field of photoelectric detection. The method includes the following steps: S1: Setting a low-noise preamplifier circuit in the photodetector as a standard detector; S2: Using a 1:1 splitter to split the laser's optical signal into two paths; S3: One path of light enters the standard detector, converting the narrow-pulse optical power into a voltage signal for detection; S4: The other path of light enters the photodetector under test for illumination testing; S5: Acquiring the photoelectric signals of the photodetector under test and the standard detector using an oscilloscope and a computer; S6: When the laser experiences momentary optical instability, the computer compensates for the reading of the photodetector under test on the oscilloscope based on the signal fluctuation amplitude fed back from the standard detector, thereby achieving accurate measurement of the AC responsivity of detector-type products.

Description

Technical Field

[0001] This invention belongs to the field of photoelectric detection and relates to a method for testing the responsivity of low-frequency narrow-pulse-width AC light. Background Technology

[0002] Optoelectronic detectors are widely used in military and civilian fields such as fiber optic gyroscopes, lidar, fuses, laser guidance, laser ranging, optical sensing, fiber optic communication, material optical monitoring instruments, material absorption spectroscopy testing, material fluorescence detection, optical power meters, optoelectronic testing, and industrial automatic control. Narrow pulse width AC responsivity has always been an important indicator for evaluating the performance of optoelectronic detector components because it can more comprehensively and objectively reflect the response characteristics of the optoelectronic detector components.

[0003] In certain practical applications of detector products, there are scenarios requiring high-speed photoelectric conversion, such as laser fuses, laser guidance, and laser aiming. Because the detection distance is very close to the target, it is necessary to sample extremely narrow pulse optical signals. Therefore, accurate measurement of narrow pulse AC responsivity is extremely important. To meet user needs, this parameter must be accurately measured. AC responsivity is the ratio of the output electrical signal amplitude to the pulse optical power. The accurate calibration of the pulse optical power value determines the measurement accuracy of AC responsivity. Since the power value of low-frequency narrow pulse optical signals is less than 10 nanowatts and the pulse width is less than 10 nanoseconds, existing optical power meters in the industry, due to insufficient sampling rates and sampling algorithms that mostly involve integration followed by smoothing, coupled with the inherent power fluctuations of the light source itself, cannot meet the calibration requirements for low-frequency narrow pulse optical power. Therefore, they cannot meet the application scenario requirement of measurement accuracy better than 2%, and are also very expensive. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for testing the optical responsivity of low-frequency narrow-pulse-width AC light.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for testing the optical responsivity of low-frequency narrow-pulse-width AC light includes the following steps:

[0007] S1: A low-noise preamplifier circuit is set in the photodetector as a standard detector;

[0008] S2: Use a 1:1 splitter to split the laser's optical signal into two paths;

[0009] S3: One beam of light enters the standard detector, which converts the narrow pulse light power into a voltage signal for detection;

[0010] S4: Another beam of light enters the photodetector under test for illumination testing;

[0011] S5: Acquire photoelectric signals of the photodetector under test and the standard detector using an oscilloscope and a computer;

[0012] S6: When the laser experiences momentary instability, the computer compensates for the reading of the photodetector under test on the oscilloscope based on the signal fluctuation amplitude fed back to the oscilloscope by the standard detector, thereby achieving accurate measurement of the AC response of detector-type products.

[0013] Furthermore, the step S1 of setting a low-noise preamplifier circuit in the photodetector specifically includes:

[0014] When photodetector chips D1 and D2 are connected in series, and a resistor R0 is connected in parallel between D1 and D2 to balance the detector structure, and a reverse voltage u1 is applied to D1, the current i1 flowing through D1 consists of photocurrent and dark current.

[0015]

[0016] Among them U T I is the voltage equivalent. s It is the reverse saturation dark current;

[0017] The photodetector chip D2 is used for light blocking, and the current i2 flowing through the photodetector chip D2 only includes the dark current component:

[0018]

[0019] According to Kirchhoff's current law, the current through resistor R0 is:

[0020] i0 = i1 - i2 (3)

[0021] Right now:

[0022]

[0023] The first part of i0 is the composite "dark current". To make it cancel out to zero, the following condition must be met:

[0024] V0=(V c +V e ) / 2 (5)

[0025] R0 is the load of the balanced detector structure, from which we can obtain:

[0026] V0 = R i PR0 (6)

[0027] If two detector chips using this structure are used, and the preamplifier circuit does not have a series resistor R0, and both PDs are simultaneously applied with equal and opposite voltages, the expression for the signal current can be obtained as equation (7):

[0028] i0 = R i P (7)

[0029] Dark current noise of the detector:

[0030] Amplifier thermal noise:

[0031]

[0032]

[0033] Where B represents the module's bandwidth, K represents the Boltzmann constant, T represents the thermodynamic temperature, and R... L This represents the equivalent output load of the module;

[0034] The noise voltage generated under these conditions is:

[0035] V n =(i n1 A k +i n2 )R L (10)

[0036] Because i n1 The reduction of [something] ultimately achieves the goal of reducing output noise.

[0037] Furthermore, step S6 specifically includes the following steps:

[0038] The detector's AC responsivity is:

[0039]

[0040] Where Vamp is the amplitude of the AC pulse voltage generated by the optical signal input detector as read from the oscilloscope, and Pin is the instantaneous pulse optical power input to the detector. The Re value of the detector is fixed under normal and constant operating conditions. Therefore, Vamp and Pin change proportionally. That is, if Vamp changes by a certain percentage, for example, increasing (or decreasing) by x%, then Pin will also change in the same direction by the same percentage—increasing (or decreasing) by x%. Therefore, if the Vamp of the standard detector changes by x%, it can be quantitatively inferred that the Pin value of the optical signal has changed by x%. When calculating the Re value of the detector under test, this change of x% is compensated into the calculation formula (11), that is, the real-time value of the optical power Pin is obtained during measurement, thereby obtaining the accurate Re value.

[0041] The beneficial effects of this invention are as follows: This invention transforms the difficult-to-calibrate optical power problem into a method for high-speed and synchronous sampling of the detector's photovoltage signal, simplifying the process and making setup easy. The test system can be built using common equipment such as an oscilloscope, signal generator, optical attenuator, and power supply. Through fine-tuning the circuitry of a standard detector, multi-band, 90dB wide-range optical power monitoring can be achieved. This detection scheme can be applied to long-wavelength detectors and the stable output monitoring of optical power from laser sources. This invention reduces preamplifier circuit noise and solves the problem of insufficient signal-to-noise ratio caused by the sacrifice of amplification due to the high-speed characteristics of detector products. It also enables accurate measurement of the AC responsivity of detector-type products.

[0042] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0043] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0044] Figure 1 Diagram of the detector access structure;

[0045] Figure 2 Schematic diagram of the AC response measurement scheme;

[0046] Figure 3 The change in the effective value of noise voltage with temperature before and after module modification;

[0047] Figure 4 This is the pulse signal output after the module has been modified. Detailed Implementation

[0048] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0049] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0050] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0051] This invention provides a method for testing the optical responsivity of low-frequency, narrow-pulse-width AC signals. Firstly, a specially designed low-noise preamplifier circuit is used to reduce the overall output background noise, thus solving the signal-to-noise ratio problem caused by the sacrifice of amplification due to the high-speed characteristics of detector products. A detector assembly using this technology is used as the standard detector for measurements in this method.

[0052] When a reverse voltage u1 is applied to the photodetector chip D1, the current i1 flowing through it consists of the photocurrent and the dark current:

[0053]

[0054] Among them U T As a voltage equivalent, at room temperature T = 300K, U T ≈26mV is a constant, I s It is the reverse saturation dark current;

[0055] Considering that D2 is used for light shielding, the current i2 flowing through the photodetector chip D2 only includes the dark current component:

[0056]

[0057] According to Kirchhoff's current law, the current through resistor R0 is:

[0058] i0 = i1 - i2 (3)

[0059] Therefore, substituting equations (1) and (2) into equation (3) and simplifying, we get:

[0060]

[0061] The first part of i0 is the composite "dark current". To make it cancel out to zero, the following condition must be met:

[0062] V0=(V c +V e ) / 2 (5)

[0063] R0 is the load of the balanced detector structure, from which we can obtain:

[0064] V0 = R i PR0 (6)

[0065] To minimize the noise signal (V0 = 0), we must make R0 = 0 and V c =-V e Therefore, if two detector chips using this structure are used, and the preamplifier circuit is not connected in series with resistor R0, and the two PDs are simultaneously applied with equal and opposite voltages, the expression for the signal current can be obtained as equation (7), thus achieving the purpose of reducing dark current.

[0066] i0 = R i P (7)

[0067] The noise of a photodetector module mainly consists of two parts: detector noise and amplifier circuit noise. Detector noise is primarily dark current noise, while amplifier circuit noise is mainly thermal noise. Detector dark current noise:

[0068]

[0069] Amplifier thermal noise:

[0070]

[0071] Where: i – dark current flowing through the detector

[0072] B - Module bandwidth usage

[0073] K – Boltzmann constant (1.38 × 10⁻⁶) -23 J / K)

[0074] T – Thermodynamic temperature

[0075] R L —Equivalent output load of the module.

[0076] The noise voltage generated under these conditions is:

[0077] V n =(i n1 A k +i n2 )R L (10)

[0078] Because i n1 The reduction of [something] ultimately achieves the goal of reducing output noise.

[0079] On the other hand, such as Figure 2 As shown, this invention uses a 1:1 splitter to split the optical path into two. One path enters a standard detector with a fast response speed, enabling narrow-pulse response optical power feedback. This involves converting the instantaneous, nanowatt-level narrow-pulse optical power into a voltage signal for detection. The other path is the test optical path for applying light to the device under test (DUT). When the laser experiences transient optical instability, the fast response capability of the monitoring detector can detect the amplitude of the signal fluctuation. Since the changes in the two optical paths are synchronized in real time, compensation is made based on the amplitude of the change when measuring the DUT. The measurement software synchronously acquires optical and electrical signals, thereby achieving accurate measurement of the AC responsivity of detector-type products with a measurement accuracy better than 2%.

[0080] The noise voltage of the module in its original state (before modification) and the opposing detector structure (after modification) was measured using a ROHED & SCHWARZ URV55 AC millivoltmeter. The noise voltage values ​​measured by the meter are RMS values. Measurements were taken at 10°C intervals inside a high and low temperature chamber. Each temperature point was held for 10 minutes until the noise voltage value stabilized before recording the noise. The test results are shown below. Figure 3 It can be seen that, regardless of whether the module state is changed, the shape of its noise-temperature curve conforms to y = Kx. 1 / 2 The noise increases more rapidly at low temperatures and more slowly at higher temperatures. The noise-temperature curve of the modified module deviates significantly from that of the original module. At the same temperature, the noise is reduced by about 0.15mV and remains almost constant. This is because the dark current entering the preamplifier circuit is greatly reduced, thus reducing the contribution of the dark current to the noise. Over the entire temperature range, the noise of the modified module is about 0.15mV lower than that of the original module, representing a noise reduction of about 30%.

[0081] Since the only change in the photodetector module was the detector connection method, and since the newly added detector chip cannot receive light signals and therefore cannot affect the light parameters, its output signal waveform is as follows: Figure 4 As shown in Table 1, no changes were observed; except for the only electrical parameter—noise, all other photoelectric parameters remained unchanged.

[0082] Table 1

[0083]

[0084] To evaluate the accuracy of the AC responsivity measurement parameters, repeated tests were performed using the same device. Since the laser's optical power naturally fluctuates during operation, Table 2 records the AC responsivity values ​​measured using the test system as the laser's optical power changes. Each measurement was spaced 5 minutes apart. It can be seen that the measurement accuracy is better than 2% after using the test system. Note: The measurement accuracy is calculated as (A1-A2)*2 / (A1+A2), where A1 and A2 are two adjacent measured values. The first and last rows were not calculated because they do not contain adjacent values.

[0085] Table 2

[0086]

[0087]

[0088] In Table 2, since the optical power fluctuates naturally with relatively small variations, to further verify the measurement accuracy of the test system, a large range of optical power variations were assumed, with 1dB intervals, ultimately resulting in a 10dB variation. The same device was used for testing, and the results were recorded. Specific measurement results are shown in Table 3. It can be seen that the measurement accuracy is better than 2%, indicating that the test system still meets the testing requirements when measuring a 10-fold change in optical power.

[0089] Table 3

[0090]

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A low frequency, narrow pulse width AC photore sponse test method characterized by: It comprises the following steps: S1: setting a low-noise preamplifier circuit in a photodetector as a standard detector; S2: using a 1:1 optical splitter to split the optical signal of a laser into two paths; S3: one path of light enters the standard detector to convert the narrow pulse optical power into a voltage signal for detection; S4: the other path of light enters the photodetector to be tested for light testing; S5: collecting the photoelectric signals of the photodetector to be tested and the standard detector through an oscilloscope and a computer; S6: when the laser appears transient light instability, the computer compensates the readings of the photodetector to be tested on the oscilloscope according to the signal fluctuation amplitude fed back to the oscilloscope by the standard detector, so as to realize accurate measurement of the exchange response of the detector product; In step S1, the low-noise preamplifier circuit in the photodetector is specifically: The photodetector chips D1 and D2 are connected in series, and a resistor R0 for balancing the detector structure is connected in parallel between D1 and D2. A reverse voltage u1 is applied to D1, and the current i1 flowing through D1 is composed of a photocurrent and a dark current: （1） where U T is the voltage equivalent, I s is the reverse saturation dark current; The photodetector chip D2 is used for light shielding, and the current i2 flowing through the photodetector chip D2 only includes a dark current part: （2） According to the Kirchhoff's current law, the current through the resistor R0 is: （3） That is: （4） The first part of i0 is the recombination dark current. To make it zero, the condition to be met is: （5） R0 is the load for balancing the detector structure, and the following can be obtained: （6） If two photodetector chips using this structure are used, the preamplifier circuit is not connected in series with the resistor R0, and the two PDs are applied with equal and opposite reverse voltages. The expression of the signal current is formula (7): （7） The dark current noise of the photodetector: （8） The thermal noise of the amplifier: （9） where B represents the module use bandwidth, K represents the Boltzmann constant, T represents the thermodynamic temperature, R L represents the equivalent output load of the module; The noise voltage generated under this condition is: （10） Due to the reduction of i n1 , the purpose of reducing the output noise is finally achieved; Step S6 specifically comprises the following steps: The exchange response of the photodetector is: （11） Where Vamp is the AC pulse voltage amplitude read from the oscilloscope due to the input of the optical signal into the photodetector, and Pin is the instantaneous pulse optical power input into the photodetector; Vamp and Pin values change proportionally. If the Vamp of the standard detector changes by x%, it can be quantitatively speculated that the optical signal Pin value changes by x%. When calculating the Re value of the photodetector to be tested, the change x% is compensated into the calculation formula (11), that is, the real-time value of the optical power Pin is obtained during measurement, so as to obtain the accurate Re value of the photodetector to be tested.

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