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J16 Series detectors are high-quality Germanium photodiodes designed for the 800 to 1800nm wavelength range. For applications where temperature stability of response is important near the cutoff, thermoelectrically cooled detectors are available.

Figure 1	Figure 2

Responsivity

A Ge photodiode generates a current across the p-n or p-i-n junction when photons of sufficient energy are absorbed within the active region. The responsivity (Amps/Watt) is a function of wavelength and detector temperature (Fig. 3). Temperature changes have little effect on the detector responsivity at wavelengths below the peak, but can be important at the longer wavelengths (Figs. 3 and 4).

Figure 3	Figure 4

For example, at 1.2 µm the change in response of a room temperature detector is less than 0.1% per °C, while at 1.7 µm the change is approximately 1.5% per °C (Fig. 4). Uniformity of response within the active region of a room-temperature Ge detector is typically better than ± 1% at 1300 nm.

Figure 5	Figure 6

Operating Circuit

The recommended operating circuit for most applications is an operational amplifier in a negative-feedback transimpedance configuration (Fig. 5). The feedback circuit converts the detector output current to a voltage, while the op-amp maintains the detector near zero-volt bias for lowest noise (see "Shunt Resistance and Dark Current").

Selection of the proper op-amp is important, as the wrong choice can add excess preamp noise or limit system bandwidth. Judson has a complete line of preamps designed to match each detector type and application. Preamp recommendations are included with the detector specifications. For high frequency applications, the detector may be reverse biased and terminated into a low impedance load (Fig. 6). Reverse biasing the detector significantly reduces junction capacitance for faster pulse response; however, the dark currents and low-frequency noise are increased.

Figure 7

Applications
Optical Power Meters
Fiber Testing
Laser Diode Control
Optical Communications
Temperature Sensors

Shunt Resistance and Dark Current: When the detector is used in the basic circuit of Figure 5, an undesirable DC offset current, or "dark current," will be produced. It is a function of the preamp input bias current Ib, the preamp input offset voltage Vos, and the detector shunt resistance RD. This total "dark current" is: Total ID = Ib + (Vos / RD)

High shunt resistance detectors will result in lowest overall DC "dark current." Preamp selection is also important; for higher shunt impedance detectors, choose a preamp with low bias current; for lower shunt impedance detectors, choose a preamp with low offset voltage (Fig. 7). When the detector is reverse biased and used in the high-speed circuit of Figure 6, the predominant dark current is a function of the applied bias voltage (Fig. 8).

Figure 8	Figure 9

Device Selection

Two key factors to consider when selecting a Judson Ge detector are
detector operating temperature and detector active area.

1. Detector Temperature: Cooling the detector reduces dark current and increases the shunt resistance RD (Fig. 9). Shunt resistance data at 25°C is listed on the specification table. The data can be applied to Figure 9 to estimate RD for detector temperatures from -40 to +60°C.

2. Active Area: Larger active areas have lower shunt resistance RD (Fig. 10), and therefore higher dark currents. When low noise is critical, the smallest detector acceptable for the application should be selected. Focusing optics may be added for increased light collection.

Figure 10	Figure 11	Figure 12

Note: For linearity to ~100mW, use the 8ND package with 20x neutral density filter in cap.

Linearity

Ge photodiode responsivity in A/W (current output per input optical power) is extremely linear with low input power levels. Response linearity is ultimately limited at high input power levels by photodiode series resistance, RS, depicted in Figure 1. Large amounts of output signal current IS can significantly forward bias the photodiode junction resulting in nonlinear output response.

Response linearity to well within ±0.04dB (±1%) is maintained with input power levels up to 15dBm at 1550nm. Power levels in excess of 15dBm result in nonlinearities as depicted in Figure 11. Both absorptive and reflective attenuation filters are available for increased high power linearity. Different levels of attenuation are available to meet any high power application.

Exceptional response uniformity is realized over the entire active area of the J16 Series Ge detectors. Typical spot scan data indicates 1300nm uniformity of response to within 1% over the entire active area.

Room Temperature Ge Detectors: J16 Series room temperature Germanium detectors are designed for operation under ambient conditions to +60°C. Judson's Germanium photodiodes have high responsivity, good linearity, fast response times, uniform response and excellent long-term stability. Please review the detailed operating information above for assistance in selecting the proper detector for your application.

Figure 13	Figure 14

Responsivity Calibration

J16 Series Ge detectors are 100% tested for minimum responsivity at 1300nm. For an additional fee, Judson will calibrate response vs. wavelength from 800 to 1800 nm (for detector size 2mm and larger only).

Device Options

Judson offers three unique Ge device options for optimum performance in different applications (Fig. 14). The "-SC" device is a p-n diode, ideal for low frequency applications and DC-average power meters. It offers the highest shunt resistance available in a Ge photodiode, resulting in the lowest DC drifts. However, its higher capacitance and low reverse bias limit make it less suitable for operation above ~1 KHz (depending on active size). The "-HS" option has a p-i-n structure for extremely low capacitance and excellent speed of response, with RD and noise similar to the standard device. This option is ideal for pulsed laser diode monitoring and general use above ~10 KHz. The standard device (no option) offers excellent performance for general use in applications from ~100Hz to 100KHz.

Preamplifiers

Recommended preamps are the Judson model PA-6 for detectors with RD less than 50Kohms, and the PA-7 for detectors with RD greater than 50 Kohms (Fig. 7). Preamps are sold separately.

Germanium Short Form Catalog in PDF Format

Packages for Ge Detectors (all dimensions in inches)

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Parallel Output Ge Arrays

Standard packaging and element configurations result in low cost and quick delivery for Judson's high-quality photodiode arrays. The 4, 16 and 32 element arrays respond to infrared radiation from 500nm to 5.0µm depending on material type. The
photodiode arrays come mounted in a dual inline package with or without a thermoelectric cooler or in a TO-66 package with three stage thermoelectric cooling where higher cooling performance is needed for thermal imaging applications. Judson's NIR arrays have a parallel output format with common substrate and one pinout for each element. This format allows for independent readings from each channel.

A separate transimpedance op-amp circuit is recommended for each channel. The Judson Model PA-7:4C, PA-7:16C and PA-7:32C preamps are convenient 4, 16 and 32 channel modules with receptacles for the array package. Transimpedance gain
is specified by the user. Heat sink modules are available for detector arrays with thermoelectric coolers.

Applications

• Clinical Analyzers
• Near-IR Spectroscopy for Analysis of:
  Protein
  Blood Samples
  Agricultural Products
• Fiber Optics:
  Far-Field Laser Diode
  Pattern Analysis
  Infrared Fiber Testing

Package: J16P Series arrays are mounted in the Judson "40P" package, a 40 pin, dual-in-line package with glass window (Fig. 15). Pins 1 and 21 are connected to the common substrate. Elements of the 16-element array are connected to pins 23-38. The 32-element array is mounted with odd-numbered elements connected to pins 3-18 and even-numbered elements connected to pins 23-38. The gap between elements is 0.01mm.

Figure 15

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J16Si Dual Wavelength "Sandwich" Detectors

Two color detectors consist of a high performance silicon detector mounted in a "sandwich" configuration over another detector. The silicon photodiode responds to radiation from 400 nm to 1000 nm. Longer wavelengths pass through the silicon and are detected by the detector underneath. J16Si Series detectors are ideal for optic power measurements that need to differentiate between 800 nm and either 1300 nm or 1550 nm. They are also useful for two-color temperature measurements. The J14SI Series are used when the temperature measurement range needs to be expanded.

Applications
- Dual-Wavelength Power Meters
- Wavelength Demultiplexers
- Pyrometers

Detector Temperature Ranges
J16Si: 500C to 2000C
J14Si: 22C to 2000C

Figure 16

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Figure 17
Dual-Wavelength Power Meter Application

Figure 18	Figure 19
Two-Color Temperature Sensor Application

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J16TE Thermoelectrically Cooled Ge Detectors
J16TE Series detectors are Judson's high-quality Ge photodiodes mounted on thermoelectric coolers for reduced dark current, improved sensitivity and superior stability. The TE coolers require less than 3W of DC power. The built-in thermistor can be used to monitor or control the detector temperature. J16TE Series detectors are mounted in TO-style packages which are filled with dry nitrogen and hermetically sealed.

J16TE1 Series One-Stage Thermoelectrically Cooled Ge
J16TE1 Series detectors are Judson's large-area Germanium detectors packaged on one-stage thermoelectric coolers. Active diameters of 10mm and 13mm allow maximum light collection. The low-cost cooler can be used at -10°C for reduced dark currents, or at higher temperatures for improved stability of response in elevated or variable ambient temperatures.

J16TE2 Series Two-Stage Thermoelectrically Cooled Ge
J16TE2 Series detectors are Ge photodiodes on high-performance two-stage coolers. DC offset current and dark current are significantly reduced at the -30°C operating temperature. These low offsets and dark currents make J16TE2 Series detectors ideal for ultrasensitive fiber optic power meters. They offer accurate measurements of optical power levels as low as -80dBm (10pW) in the DC mode and -90dBm (1pW) with an optical chopper and lock-in amplifier.

Figure 20 DC Offset Current vs Temp (near 0V bias)	Figure 21 Dark Current vs Temperature

Thermoelectric Cooler Operation
The figures below show typical TE1 and TE2 cooler power requirements. A simple convection heat sink is required for maximum cooling.

Figure 22 Detector Temp. vs TE1 Cooler Current	Figure 23 Detector Temp. vs TE2 Cooler Current

The following figure shows the effect of heat sink temperature on J16TE2 detector temperature.

Figure 24

Preamplifiers
The PA-7 preamplifier offers DC stability, low noise, adjustable gain and wide bandwidth (DC to 50KHz). The PA-9 fixed-gain preamplifier offers lowest noise at higher frequencies (1KHz to 100KHz). At high frequencies, the detector capacitance and preamp voltage noise contribute significantly to the system noise. (See figure 25.)

Figure 25

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J16A Ge Avalanche Photodiodes (APDs)

The J16A Series Germanium Avalanche Photodiodes are designed for high-speed applications at 800 and 1300nm. Judson APDs offer low dark currents and bandwidths up to 1.5GHz with active sizes of 100µm and 300µm diameter. The J16A Series APDs have undergone extensive reliability testing. Reliability has been demonstrated to be better than 10 FITs corresponding to less than 1% failure rate over 20 years service. Reliability data available upon request.

Applications
- Local area networks
- OTDRs
- Transmission systems

Multiplication Characteristics
An internal gain mechanism makes the J16A the solid state counterpart of the photomultiplier tube. This internal gain is known as the Multiplication Factor (M) and is a function of the reverse bias voltage VR applied to the diode. (See Fig. 26.)

Breakdown Voltage and Dark Current
The avalanche breakdown voltage VB is the reverse bias voltage at which the diode's dark current becomes infinite. In practice, the dark current used to define breakdown voltage is 100µA. (See Fig. 27.)

Cutoff Frequency
The cutoff frequency fc is the frequency at which the output signal power is down by 3dB. In the high multiplication region, the product of M and bandwidth becomes a constant, called the gain-bandwidth product, and cutoff frequency decreases with increasing M. (See Fig. 28.)

Figure 26	Figure 27

Figure 28

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J16D Liquid Nitrogen Cooled Germanium Detectors

The J16D Series Ge detectors offer the ultimate sensitivity for 800 to 1400 nm detection. Cooling the Ge photodiode to 77°K results in extremely high shunt impedance for Noise Equivalent Power (NEP) typically below 0.01 pW/Hz1/2.

Applications
- Fiber Testing
- NIR Spectroscopy

Dewar Packages
J16D detectors are packaged in glass or metal dewars with sapphire windows. J16D detectors have extremely high shunt impedance RD and therefore very low intrinsic noise. When used in environments where vibration is present, the microphonic noise from the dewar leads may dominate the detector noise. Under these conditions, a glass dewar is recommended, as the leads are imbedded in the glass and immune to vibrations. Care must be taken with external connections to avoid noise from vibrations outside the dewar. Metal dewars are suitable for other applications and can be periodically re-evacuated.

Preamplifiers and System Noise
Optimum J16D detector performance is achieved with Judson transimpedance gain preamplifiers. The PA-9 or PA-7 preamplifier converts the detector output current to a voltage, while maintaining the detector at the optimum zero volt bias. The PA-9 fixed-gain preamp is specifically matched to each detector to provide maximum sensitivity, gain and bandwidth. The PA-7 preamp offers adjustable gain and is suitable for DC and low-frequency applications. At high frequencies, the detector capacitance and preamp voltage noise contribute significantly to the system noise.

Figure 29 J16 Detectivity vs Wavelength	Figure 30 J16D Noise Equivalent Power vs Frequency

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