Choosing a Detector or Receiver
How do you choose a detector or receiver?
When choosing a detector or receiver, the most important factor may be bandwidth, impulse response, or sensitivity: depending upon your particular application. For instance, if you have a very weak signal, conversion gain (often called sensitivity) may be the most important specification. The value we list in the characteristics table is the peak value. Keep in mind that conversion gain is a function of wavelength, and is directly proportional to the responsivity. So to find the value at the wavelength of interest, you will need to look at the responsivity curve provided for each detector. For example, for the Model 1601-AC, the responsivity at 500 nm is about 50% of its peak value around 750 nm. Thus, the conversion gain at 500 nm is about 180 V/W: half of its typical maximum conversion gain. Another important factor with weak signals is the minimum noise-equivalent power (NEP). This parameter can be used to calculate the smallest signal that can be observed with a given detector at a given bandwidth. (For calculating the minimum detectable signal, see the section on characteristics.)
For applications requiring either high resolution in the time domain or wide frequency bandwidths, you'll need a fast detector or receiver. In both cases, you'll want to start by making sure that you choose a device with a 3-dB bandwidth that is much greater than that of your signal. If the 3-dB bandwidth is comparable to the highest significant frequency component of the signal you need to measure, the detector or receiver will introduce unacceptable distortion in your measurement. A good rule of thumb is that the bandwidth should be 2-3 times faster than that of the signal you want to measure. In addition, you'll need to consider whether you need a flat-frequency response or a clean ring-free time-domain response. For time-domain applications where accurate reproduction of your signal is paramount, choose one of our time-domain-optimized detectors, which were specifically designed to provide an impulse response with minimal ringing. For further information, see High-Speed Detector and Receiver FAQs.
What's the difference between a detector and receiver?
We distinguish receivers as photodetectors with built-in amplifiers. The detectors that we have can be modeled as a capacitor and resistor pair. The smaller the diameter, the smaller the capacitance, thus the faster the bandwidth.
Can I use the receivers and detectors as power meters? How accurate are the responsivity curves?
We do not recommend using our receivers and detectors for accurate power measurements. Unfortunately, responsivity curves do vary from detector to detector, and since no experimental conditions are ever the same, it is impossible to determine the responsivity curve for one detector and use it as a reference for every detector. The conversion gain (based on the responsivity) is not precise since this number will vary with wavelength.
Can I use an ST connector with the FC version?
In our high-speed models, you have the option of an ST or FC connector. Most of our receivers and detectors, however, have only FC connections. With these, you can easily adapt the input or output using a fiber patch cable or an FC-to-ST converter from companies such as:
E-mail: email@example.com or firstname.lastname@example.org
Can I use an APC connector with the "-FC" low-speed receivers?
Yes, you can use an APC connector with the "-FC" low-speed receivers. However, because the photodetector packaging was designed specifically for FC-PC types, you'll get a 20% reduction in responsivity.
Note: that this is not applicable to the Model 15XX or any of the Model 14XX and 10XX high-speed detectors.
Can I use a multimode fiber with the detectors and receivers?
You can use multimode fiber with our low-speed receivers (<6 GHz) with some loss in responsivity due to less efficient optical coupling. However, with the 6-GHz photoreceivers and 25-GHz photodetectors you will need to order the "-50" multimode option. In these versions, the internal single-mode fiber pigtail is replaced with a 50-µm core fiber. A GRIN lens is used to focus the light onto the 25-µm diameter active area. Our high-speed detectors do not have this option mainly because the detectors are much smaller.
What's the NEP? How is it related to the minimum detectable signal?
The noise-equivalent power (NEP) is used as a figure of merit for the weakest optical signal that can be detected. It is the root mean square of the optical power that will produce a signal-to-noise ratio (SNR) of 1 in a 1-Hz bandwidth. We measure this by first measuring the voltage noise at the output of our detectors. This linear spectral density has units of VHz-1/2. We convert this to an equivalent optical noise by dividing by the responsivity (A/W) and the transimpedance gain (V/A). This yields a NEP with units of WHz-1/2.
The minimum optical power defined as the optical power required to produce a SNR of 1 for the entire measurement bandwidth (B) can be expressed as:
As shown in the above relationship, limiting your measurement bandwidth with band-pass filters significantly reduces the amount of noise in your measurement.
Often the NEP is frequency-dependent. For example, the NEP for the Model 1801 is 3.3 pWHz-1/2 from DC to 10 MHz, and 30 pWHz-1/2 from 10 MHz to more than 30 MHz. Therefore, without any band-pass filtering the minimum detectable signal is 322 nWrms.
In other words, 322 nanowatts is the optical power that you must put into the 1801 in order to have an SNR of 1. The 322 nWrms of equivalent optical noise translates into an output voltage noise of: (322 nanowatts)·(2x104 V/W) = 6.4 mVrms. The peak-to-peak voltage noise is approximately 6-8 times larger than the rms voltage noise.
If, for example, you filter the bandwidth of the 1801 at the output to 20 MHz, then the equivalent optical noise is reduced to: 95 nW and the output voltage noise is 2.28 mVrms.
How do I calculate the minimum detectable optical power?
The minimum detectable optical power (Pmin) is a function of wavelength (l), measurement bandwidth (B), and noise-equivalent power (NEP). Pertinent parameters can be found in our specification tables and graphs, with the exception of measurement bandwidth. The measurement bandwidth is determined by your choice of output filtering. If you choose not to use any output filtering, the photoreceiver's 3-dB frequency is a good approximation of measurement bandwidth.
Rmax is the maximum responsivity of the detector, R(l) is the responsivity of the detector at the wavelength of interest, and NEP is given in the characteristics tables.Typical Responsivity: Model 1601-AC (Blue) Model 1611-AC (Black)
For an example, we can calculate the minimum detectable optical power of a Model 1611-AC receiver at 1.0 µm using an output filter with a 10-kHz bandwidth. From the specification table and responsivity graphs, Rmax is 1 A/W, and R at 1.0 µm is 0.7 A/W. Therefore, NEP at 1.0 µm is 28.6 pWHz-1/2 and Pmin is 3 nW.
What's the dynamic range of the receivers or detectors?
The dynamic range is the ratio of the maximum power you can detect over the minimum power you can detect. This provides you with an operating range over which you can detect power changes. This value is typically given in dB.
For the Model 1607-AC a good rough estimate of the dynamic range is:
Maximum detectable power (MaxDP) = 2 mW (saturation power)
Minimum detectable power (MinDP) = 1µW (min NEP)
Min detectable power = (min NEP)· (BW-1/2)
In dB, Dynamic Range = 10log(MaxDP/ MinDP) = 10log(2000) for an approximate Dynamic Range of 33 dB.
What determines the Saturation Power in the receivers (detectors with amplifiers)?
The value given for the saturation power is determined by the maximum photocurrent that can go into the RF amplifier before saturation. It is also stated as the maximum responsivity. Therefore, if you're using the receivers at a wavelength other than at maximum responsivity you will be able to input a higher optical power as proportional to the decrease in responsivity. For example, with the Model 1617 high-speed balanced receiver, the saturation power is 1 mW at 1600 nm. At 950 nm, the responsivity is 0.5 A/W versus 1.0 A/W at 1600 nm. Therefore the saturation power is 2 mW. Also, note that with these balanced receivers, since the saturation power is due to the current into the RF amplifier, you are limited by the difference photocurrent. Thus, you can put more than the saturation power on each photodetector as long as the difference between the two powers is less than the saturation power stated in the specification table.
In the receivers,what limits the Absolute Maximum Input Power?
The Absolute Maximum Input Power is determined by the input current at which the RF amplifier will become damaged, and it is stated at the wavelength where the responsivity is at its maximum. Therefore, if you're using the receivers at a wavelength other than at maximum responsivity you will be able to input a higher optical power as proportional to the decrease in responsivity.
General Purpose Detectors
When do I need an AC-coupled receiver?
Because an AC-coupled receiver has no response at DC, these receivers are great when you have a small modulation on a large cw component (such as the relative intensity noise (RIN) measurements of a laser). An AC-coupled version will be insensitive to the large cw component, which, in a DC-coupled version, would saturate the receiver's internal amplifier.
Typically, we specify a 3-dB low-frequency cutoff, below which there is very little response. For ease of use, all our AC-coupled photoreceivers are equipped with a DC-bias monitor that outputs a voltage proportional to the DC photocurrent flowing through the photodetector. This feature enables you to verify the operation of your detector and facilitates alignment of our free-space detectors. You can also use it for normalization or to determine the modulation depth of your signal.
NOTE: Since the AC-coupled receivers are insensitive to frequencies below the low-frequency cutoff, there will be some pulse distortion if your signals have significant low-frequency components. For example, with a 30-kHz low-frequency cutoff, as in our Model 16XX receivers, the longest transition that can be measured is approximately 6 µs. If you have pulses longer than this, you will see "drooping."
What cables come with my receiver?
For photoreceivers that operate from a power supply, the power connector on the photoreceiver is a 3-pin shielded microconnector. Each receiver comes with two different power cables: a Model 0924 banana-plug-to-pico (m8) cable and a Model 0923 pico (m8)-to-pico (m8) cable. If you have a New Focus Model 0901 power supply, use the Model 0923 cable on one of the supply's 0.3-A microconnector outputs only. Use the Model 0924 cable with power supplies other than the 0901 that provide a minimum of 0.3 A of current at ±15 V. Photoreceivers with AC-coupled outputs are also supplied with an SMB-to-BNC cable for use with the SMB output of the bias monitor.
What type of power supply do I need?
Some New Focus receivers, such as the 20X1, 203X, and 215X, operate on batteries. Other receivers, including the 20X7, 16X1, and 18X1, operate from a ±15-V power supply. You can use a general-purpose laboratory supply, but we recommend the Model 0901 power supply with its protective circuitry.
What is the output impedance of the Model 215X femtowatt receiver?
The Model 215X femtowatt photoreceiver has an output impedance of 100 W and a bandwidth of 750 Hz. For this sort of low-frequency measurement, the 100-W output impedance of the 215X does not need to be matched to the input impedance of the instrument that it is connected to. In fact, the receiver was designed to be connected to an instrument (such as an oscilloscope or a voltmeter) that has a high input impedance. Typically, the input impedance of these instruments is 1 MW . You can also use the Model 215X with an instrument that has a relatively low input impedance of 50 W or 1 KW. In the case of a 50-W input impedance, the voltage that you measure will be about 1/3 of the output voltage of the 2151. The reason for this is that the 100-W output impedance of the 215X and the 50-W input impedance of your instrument form a voltage divider.
With the Model 2XXX receivers, how do I estimate what the output signal strength will be?
For these models, the output voltage versus input laser power is calculated from the equation Vout=Pin·G·R, where Vout is the output voltage, Pin is the input optical power in mW, G is the gain factor (determined by the setting of the gain knobs and switches), and R is the response factor in volts/mW. Keep in mind that R varies with wavelength. For example, for the Model 2001, if you input 10 mW of optical power at 532 nm with the 2001 set to x3 gain, the output voltage will be approximately: Vout=(10 mW)·(3)·(0.2 V/mW)=6 V. Please note that this calculation is approximate, and the voltage that you actually measure can vary by about ±20% because the response factor of each photodiode is slightly different and the gain factor can vary from one Model 2001 to another.
Can I use a multimode fiber with a 50-µm core with the Model 18XX-FC receivers?
These receivers will work fine with a 50-µm core fiber. For the Model 1801-FC, the photodetector diameter is 0.9 mm, and the distance from the end of the fiber to the surface of the photodiode is about 3.5 mm, so there should be no problem collecting the light from a 50-µm core fiber. For the Model 1811-FC, the responsivity of the photodiode will decrease by about 10% as compared with a single-mode fiber because not all of the light will be captured by the photodiode. It is not recommended to use a multimode fiber with a core diameter greater than 50 µm.
What's the estimated bandwidth for the Model 162X nanosecond detector?
We do not specifically specify the bandwidth of the 162X. You can estimate, however, that on the 50-W setting the bandwidth is approximately 400 MHz. On the 10-kW setting, the bandwidth is approximately 1 MHz or greater. On the "Open" setting, the customer supplies an external load resistor. Since the bandwidth of the 162X depends on the size of the load resistor, we cannot specify the bandwidth in this mode.
Can I use a 1-MW input on the oscilloscope with the Model 162X nanosecond detector and the Model 203X large-area receiver?
Yes, you can use this input for both the Model 1621(max speed 1 GHz) and the Model 2031 (max speed 1 MHz). Keep in mind, however, that with a faster input signal (on the order of tens of nanoseconds), the impedance mismatch may cause some additional ringing or a very minor loss in signal strength for the Model 1621. For a low-frequency measurement (1 MHz or less), either of these units will work fine with a 1-MW input impedance oscilloscope. For signals faster than 1 to 10 MHz, it is most efficient to impedance match the output of the Model 162X with the oscilloscope and cable (50 W). This results in minimal distortion of your input signal and maximum efficiency.
Can I use a voltmeter or the 1-MW input on the oscilloscope with the Models 1601, 1611 and 18XX receivers?
Both of these receivers have a 50-W output impedance, and transimpedance-gain specification listed assumes that you are connected to a 50-W scope input. If you measure the output voltage with a voltmeter or a 1-MW oscilloscope, then the transimpedance gain doubles. Keep in mind, however, the impedance mismatch may cause some additional ringing.
What is a balanced receiver?
Balanced receivers work by subtracting the photocurrent from two well-matched photodetectors. Common-mode noise that is present on both the reference and signal beams (such as laser-intensity noise) is cancelled out and thus doesn't appear as part of the signal. On the other hand, any imbalance between the photocurrents generated by the reference and signal detectors, whether intentional or unintentional, is amplified and is seen as the signal. So with these balanced receivers, you can cancel out laser-intensity noise in any experimental setup that produces a reference signal achieving shot-noise-limited performance without the need for lock-in amplifiers. Applications include spectroscopy, ellipsometry, and heterodyne-detection experiments. (For more information download Application Note 7 on FM spectroscopy.)
What does the Loop Bandwidth knob do in the Model 20X7 Nirvana auto-balanced receiver?
The Loop Bandwidth knob allows you to set the cutoff frequency of the electronic gain compensation circuit. If you want to compensate for low-frequency fluctuations, then you can set the Loop Bandwidth to a low setting. Otherwise, we generally recommend operating with the Loop Bandwidth set to maximum.
Can I use multimode fiber with the FC versions of the Model 20X7 Nirvana auto-balanced receivers?
Yes, because the detectors are fairly large (2.5 and 1.0 mm), you can use almost any fiber.
Why is the Model 20X7 called the "Nirvana" balanced receiver?
Nirvana is a state of enlightenment characterized by an absence of the "noise" of worldly distractions. We named our auto-balanced receiver "Nirvana" because its auto-balancing circuit lets you see the light even with the noisiest of lasers.