Bias tee

A bias tee has a DC port, an RF port and a combined RF+DC port. An inductor connects the DC port to the combined port and passes the bias current while blocking high frequencies; a capacitor connects the RF port to the combined port and passes the signal while blocking DC. The result is a single coaxial line that carries both: a laser diode can be driven with a steady current plus modulation, and the AC part of a photocurrent separated from its DC part. Broadband coaxial bias tees typically cover from tens of kHz up to tens of GHz, with an insertion loss of a fraction of a dB to a couple of dB across that range and a DC current rating of a few hundred mA to a few amperes, depending on the inductor.

Uses on the optical bench

• Laser modulation. A current source on the DC port sets the operating point above threshold, and a signal generator, pattern generator or the port of a network analyzer on the RF port adds the small-signal or large-signal modulation of a directly modulated laser.
• Photodetector readout. A photodiode or avalanche photodiode receives its reverse bias through the DC port, often from a source-measure unit that also reads the average photocurrent, while the RF port delivers the signal to an amplifier, oscilloscope or electrical spectrum analyzer. This split is the standard front end for relative intensity noise and photodetector bandwidth measurements, where the DC photocurrent normalizes the noise spectrum.

Corner frequencies

The low-frequency limit comes from both elements. The series capacitor and the resistance around it form a high-pass filter, and the shunt inductor bypasses the signal to the (RF-grounded) DC port at low frequencies. For a 50 Ω source driving a 50 Ω load, the capacitor sees $R = 100$ Ω in series and the inductor sees the two 50 Ω ports in parallel, 25 Ω:

$$f_C = \frac{1}{2\pi R C}, \qquad f_L = \frac{R_\parallel}{2\pi L}.$$

With $C = 100$ nF the capacitor corner is 16 kHz; with $L = 100$ μH the inductor corner is 40 kHz. A circuit calculation of the combined network gives a loss of 3.0 dB at 40 kHz, 0.3 dB at 100 kHz and 0.0025 dB at 1 MHz, so in this example the inductor sets the lower edge. The two time constants are $RC = 10$ μs and $L/R_\parallel = 4$ μs, and their droops add, so a run of 31 identical bits at 2.5 Gb/s (12.4 ns) droops by about 0.4%, but slow framing patterns or a low-frequency spectrum can produce visible baseline wander in an eye diagram.

The upper limit is set by parasitics. A single 100 μH inductor with 1 pF of winding capacitance self-resonates at about 16 MHz, above which it behaves as a capacitor and stops isolating the DC port. Broadband bias tees therefore use a chain of inductors of decreasing size, or a conical inductor; the response shows small ripples where one section hands over to the next.

Characterizing a bias tee

Transmission, return loss and RF-to-DC isolation are measured with a vector network analyzer, with the DC port terminated as it will be in use (a supply is a low impedance at RF, an open port is not). In precise frequency-response measurements of a laser or detector, the bias tee's S-parameters are removed along with those of cables and probes by de-embedding.

Pitfalls

• Current rating and DC resistance. The inductor winding has some resistance; at 60 mA a winding resistance of 1 Ω drops 60 mV, which a voltage-mode supply does not correct. Exceeding the current rating saturates the inductor core and lowers its inductance, raising the low-frequency corner.
• Charge on the capacitor. Connecting a charged RF cable or an instrument output with a DC offset to the RF port couples a transient straight into the device.
• Hot-plugging and surges. Connecting or disconnecting the device while the bias is on produces an inductive kick when the current is interrupted; laser diodes and APDs are damaged by such transients and by electrostatic discharge. Ramping the bias to zero before changing connections, and handling the device with ESD precautions, prevents most such failures.
• Ground loops. The bias supply, the RF instrument and the device share the coaxial shield, so mains-frequency currents can circulate between instrument grounds and appear as low-frequency noise.

Common questions

What is the difference between a bias tee and a DC block?

A DC block is a series capacitor alone: it passes RF and stops DC but provides no way to apply bias. A bias tee adds the inductive DC path, so it is a DC block plus a bias injection port.

Which direction does a bias tee go?

It is passive and reciprocal for RF, so signal can flow from the RF port to the combined port (driving a laser) or from the combined port to the RF port (reading a photodiode). The combined port always faces the device that needs the bias.

Can a bias tee pass a square wave or a slow pulse?

Only the part of its spectrum above the low-frequency corner. A pulse of duration $T$ droops by roughly $T/RC + T R_\parallel/L$, the sum of the two contributions, where $R_\parallel$ is the source and load resistances in parallel, so long pulses need a driver that delivers the full current directly.

References: D. M. Pozar, Microwave Engineering, 4th ed. (Wiley, 2012); L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012).