balanced photodetection (original) (raw)

Definition: a method of photodetection which is sensitive to differences in optical powers but not to common noise

Alternative term: differential photodetection

Categories: photonic devices, light detection and characterization, fluctuations and noise, methods

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Related: optical heterodyne detectionphotodetectorstelecom receiversshot noisesignal-to-noise ratioelectronics for photodetectionsqueezed states of light

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Contents

Principle of Balanced Detection

The method of balanced photodetection (or differential photodetection) has been developed for detecting small differences in optical power between two optical input signals while largely suppressing any common fluctuations in the inputs.

Figure 1: A simple electronic circuit for balanced photodetection. Ideally, the transimpedance amplifier should keep the potential of its input such that both photodiodes are operated with the same bias voltages.

In a simple form, one uses two photodiodes connected in series, so that their photocurrents cancel each other when they are equal (see Figure 1). The difference in photocurrents is sent to a transimpedance amplifier, which produces an output voltage proportional to that difference.

Different electronic circuits are used, e.g. with separate transimpedance amplifiers for the two signals and subsequent difference generation — and possibly producing an output for the sum of photocurrents in addition. Auto-balancing (see below) is also an important feature of enhanced circuits.

As an example of an application, we can consider the measurement of weak absorption features in spectroscopy. Here, one photodetector is used for light which is transmitted through a spectroscopic sample, while the other photodetector is illuminated with light from the same source but which has not gone through the sample. (The two light beams are obtained with a beam splitter between the light source and the sample.) Any intensity noise of the light source is largely canceled out with that balanced detection scheme, so that measurements with an improved signal-to-noise ratio can be done.

The mentioned cancellation is called common mode rejection; see below for more details.

Technical Details

Types of Photodetectors

Most balanced detectors use photodiodes, in particular p–i–n photodiodes. These offer high linearity and a high detection bandwidth (sometimes many gigahertz). Most common are silicon-based detectors, but other materials are available for longer wavelengths. See the article on photodiodes for more details.

Common Mode Rejection

The mentioned cancellation is called common mode rejection and can be quantified as a common mode rejection ratio (CMRR) in decibels, based on the ratio of signal powers. In practice, CMRR values of 20–40 dB are typical over useful bandwidths; with careful balancing and suitable hardware, >50 dB is achievable. Optimization concerns aspects like differences in responsivity, series resistance and capacitance of photodiodes, optimized beam alignment as well as details of the electronics.

Although a balanced detection setup can be assembled from single components, it can be helpful to use a prefabricated complete assembly, containing matched photodiodes (see below) and the required electronics.

Perfect cancellation would require a perfect match between the two detectors including the optical paths. There are different approaches to optimize a setup in that respect:

• One may use matched photodiodes, having very similar properties in terms of quantum efficiency, frequency response, etc. They may be made on the same semiconductor chip in the same process. However, this approach does not help against optical asymmetries.
• One could use an adjustable optical attenuator for one of the beams, which may also compensate for any difference in responsivity between the two detectors. However, that would deteriorate the quantum efficiency of detection.
• There are auto-balanced photodetectors, where an electronic circuit automatically cancels asymmetries at low frequencies. In a typical realization, one uses an electronically controlled current splitter (realized with a pair of bipolar transistors) for one of the photodiodes, which generates a significantly higher photocurrent. The current splitter can proportionally reduce that current, reaching a current balance. For that purpose, it is automatically adjusted with a feedback circuit of limited bandwidth. The actual measurement can then be done only for frequencies above that feedback bandwidth. The auto-balancing allows one to achieve a much improved CMRR.

Figure 2: A fiber-coupled high-speed balanced InGaAs detector as used in OCT systems. Source: Wieserlabs Electronics

Note that problems can arise e.g. when a free-space input beam extends to the edges of the light-sensitive region on a detector: Small changes in beam position may then translate into significant photocurrent changes which affect the difference signal. Deviations from perfect uniformity of the detector responsivity may cause similar problems in conjunction with changes in beam position. Such problems cannot occur for fiber-coupled photodiodes with single-mode fibers, except when free-space light needs to be coupled into such fibers, where position-dependent coupling occurs similarly. All-fiber setups, particularly when made with polarization-maintaining single-mode fibers, can be optimized for particularly high common mode rejection ratio (CMRR).

Any asymmetries in the electronics should also be avoided. Particularly for high-frequency detection, it is also important to have precisely matched cable lengths, since otherwise one would introduce frequency-dependent phase shifts.

Some balanced detectors have an additional low-frequency DC-coupled monitor output which can be used to check the balance of photocurrents.

Shot Noise and Electronic Noise

In some respects, the behavior of quantum noise differs from that of classical intensity noise. Even when using a perfect 50:50 beam splitter, the optical power fluctuations in the two output ports of such a device are not fully correlated with each other. (In an intuitive picture, photons randomly decide for one of two paths, which introduces noise.) Therefore, even a perfectly balanced detector, applied to the two outputs of the beam splitter, would produce some output fluctuations which are related to shot noise, and this often limits the achievable signal-to-noise ratio. This quantum noise limit is still much lower than the noise level achievable in many cases without balanced detection.

The relative intensity noise at the shot noise level increases as the optical power is reduced. Therefore, one will often try to operate the photodetectors with power levels as high as possible, limited either by the available optical powers or by the power handling capability of the detectors (also considering detector saturation).

Electronic noise also contributes to the measured noise. For measurements on shot-noise limited light (e.g., laser light at high enough noise frequencies), electronic noise should be at least somewhat below the shot noise level in order not to contribute substantially. For sub-shot-noise measurements (with squeezed states of light), a lower electronic noise level is needed. In that respect, detectors are characterized by their dark noise clearance in decibels based on the ratio of shot noise and electronic noise powers. Generally, detectors with very low noise-equivalent power are preferred.

Features of Available Balanced Detectors

Available devices for balanced photodetection can be of different types, offering different features:

• There are compact printed circuit boards (PCBs), containing a difference amplifier and either two matched photodiodes (possibly connected with fiber-optic adapters or receiving light from free space) or two electronic inputs for connection to external photodiodes. Such PCBs can be integrated as OEM parts.
• There are also fully packaged balanced detectors, including a low-noise power supply.
• Outputs can be DC or AC-coupled. There can also be a DC-coupled monitor output, allowing one to check whether the photocurrents are balanced. Some circuits auto-balance photocurrents, as explained above.
• Some devices offer an adjustable gain, either continuously or in steps.

Applications of Balanced Photodetection

Absorption Spectroscopy

As already mentioned above, one may apply balanced detection in laser absorption spectroscopy. The problem with direct detection (using a single photodetector) would be that the detector cannot distinguish between fluctuations due to laser noise and those caused by absorption features. A balanced photodetector can largely suppress the influence of laser noise, which is a common input for both ports — particularly if the setup is well balanced.

Balanced detection can also be applied in conjunction with frequency modulation spectroscopy, where absorption features cause slight changes in amplitude and phase of a laser beam, which can be revealed by comparison with a reference beam in a balanced detector. Of course, the detector should be able to temporally resolve the modulation.

Similar methods can also be applied for pump–probe measurements. The inputs to the balanced detectors are the probe beam and a reference beam derived from it, which is not affected by the investigated sample.

Homodyne Detection of Laser Noise

Balanced photodetection is often employed after a 50:50 beam splitter to analyze the intensity noise of a laser or to characterize squeezed states of light. The two photodiodes in the balanced detector generate photocurrents that are electronically combined to yield both their sum and difference signals, which can be analyzed using a spectrum analyzer.

The sum signal corresponds to the total optical power incident on the detector pair and therefore reflects the laser's intensity noise, similar to what would be measured with a single photodiode. The difference signal, on the other hand, ideally cancels common-mode fluctuations (such as laser intensity noise) and isolates the quantum shot noise contribution. This enables a direct comparison between the actual laser noise and the fundamental shot-noise limit.

In systems using amplitude-squeezed light, the difference signal can fall below the shot-noise level, indicating genuine noise reduction due to quantum correlations in the light field. Accurate detection of this sub-shot-noise behavior requires detectors with low electronic dark noise and high common-mode rejection to ensure that technical noise does not obscure the quantum signal.

Homodyne detection can also be extended to quadrature measurements of light fields. In such setups, a local oscillator (LO) — a strong coherent field derived from the same laser or a phase-locked source — is mixed with the signal on a beam splitter. By adjusting the relative phase between the LO and the signal, one can selectively measure different optical quadratures (amplitude or phase). This makes homodyne detection a cornerstone technique in quantum optics and continuous-variable quantum information processing.

Heterodyne Detection

Heterodyne detection is a related but distinct technique in which the local oscillator (LO) is deliberately frequency-shifted relative to the signal field, typically by a radio-frequency (RF) offset using an acousto-optic or electro-optic modulator. When the two fields interfere on a balanced photodetector, the resulting photocurrent oscillates at the beat frequency corresponding to the difference between the LO and signal frequencies.

This beat signal carries both the amplitude and phase information of the optical field and is shifted from direct current (DC) to an RF frequency. Because most detector and electronic noise sources — such as ($1/f$) (flicker) noise and amplifier noise — are much smaller at higher frequencies, the signal-to-noise ratio of the measurement can be significantly improved. The heterodyne technique thus translates the optical information into a frequency range where technical noise is reduced and electronic signal processing (such as filtering and demodulation) can be performed with high precision.

By demodulating the RF signal, one can extract both the in-phase (I) and quadrature (Q) components, providing full access to the complex optical field. In contrast to homodyne detection, which measures only one optical quadrature determined by the LO phase, heterodyne detection effectively measures both quadratures simultaneously. However, this comes at the expense of an additional 3 dB of vacuum noise, which arises because both upper and lower optical sidebands contribute to the detected signal.

Heterodyne detection is widely used in telecom receivers for coherent optical data transmission, frequency metrology, and in quantum optics experiments that require precise measurement of optical phase or frequency fluctuations. It is particularly advantageous when the signal spectrum is narrow and technical noise dominates at low frequencies.

Interferometric Detection for OCT, Sensors and LIDAR

In various kinds of interferometers — for example, those used for optical coherence tomography, for fiber-optic sensors, or for coherent LIDAR — one can substantially improve the noise performance with balanced detection. The two photodiodes of the balanced detector receive light from the two outputs of the beam splitter where the signal beam is superimposed with the reference beam. The difference of the photocurrents delivers the interferometric signal while suppressing intensity noise of the light source.

Such techniques are also used for optical heterodyne detection in various other applications.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is balanced photodetection?

Balanced photodetection is a technique used to detect small differences in optical power between two light beams. It employs two photodetectors and an electronic circuit that subtracts their signals, which largely suppresses any noise common to both beams.

What is the main advantage of balanced detection?

Its key advantage is the high 'common mode rejection', meaning it effectively cancels noise present on both optical inputs, such as laser intensity fluctuations. This significantly improves the signal-to-noise ratio of a measurement.

How does a balanced detector work?

In a simple configuration, two photodiodes convert two optical beams into photocurrents. A differencing circuit, often a transimpedance amplifier, then generates an output voltage proportional to the difference between these two currents.

What is the common mode rejection ratio (CMRR)?

The Common Mode Rejection Ratio (CMRR) is a figure of merit, specified in decibels (dB), that quantifies how effectively a balanced detector suppresses common-mode signals relative to differential signals. Higher values indicate better performance.

Can a balanced detector eliminate all optical noise?

No. While it is very effective against classical intensity noise, it cannot fully cancel quantum noise (shot noise). The statistical nature of photon detection at a beam splitter introduces noise that is not perfectly correlated between the two detector arms.

What is an auto-balanced photodetector?

An auto-balanced photodetector uses an electronic feedback circuit to automatically nullify any DC or low-frequency imbalance between the photocurrents. This allows it to achieve a very high common mode rejection ratio without requiring perfectly matched optical powers.

What are common applications for balanced detectors?

They are widely used in sensitive applications like absorption spectroscopy, pump–probe measurements, optical coherence tomography (OCT), fiber-optic sensors, coherent LIDAR, and for characterizing laser noise or squeezed states of light.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 16 suppliers for balanced photodetectors. Among them:

⚙ hardware

The ULN-PDB module is a plug-and-play ultralow noise balanced photodetector in a compact and user-friendly package. It offers the best performances in terms of signal-to-noise ratio. ULN-PDB is proposed with InGaAs or Si photodiodes (with FC connectors) and offers a bandwidth of 100 MHz (adjustable on demand) with a high gain of 39 kV/A (adjustable on demand) in a DC- or AC-coupled configuration.

⚙ hardware

Our balanced detectors are engineered for quantum optics applications with low electronic noise and high quantum efficiencies. The on-board demodulation circuit offers the possibility to generate error signals and implement locking loops for the experiment or application. Available standard wavelengths include 1064 nm and 1550 nm. Other wavelengths and fiber coupling are possible upon request. Visit our website to learn more and see how our technology can support your research.

⚙ hardware

ALPHALAS offers balanced photodetectors with bandwidths from DC up to 10 GHz and wavelength ranges from 170 nm to 2600 nm. Both free-space and fiber-coupled models are available.

⚙ hardware

FEMTO offers low-noise balanced photoreceivers for wavelength from 320 nm to 1700 nm and bandwidths up to 500 MHz. The modules have an exceptionally high common mode rejection (CMRR) of up to 55 dB. Fiber-coupled and free space versions are available.

Bibliography

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