Directly modulated laser (DML)

A directly modulated laser (DML) is a semiconductor laser whose drive current carries the data signal: the current swings between two or more levels above threshold, and the optical power follows. It needs no external modulator, so it is the simplest, cheapest and most power-efficient type of optical transmitter. Single-mode DFB lasers are directly modulated in 1310 nm datacenter and access links, and multimode VCSELs in 850 nm links inside data centers. Fast DFBs reach small-signal bandwidths of about 20–30 GHz, enough for 25–50 Gbaud with on-off keying or PAM4. The cost is frequency chirp, which interacts with fiber dispersion and limits reach, most severely at 1550 nm.

Bandwidth and the relaxation-oscillation frequency

The modulation response of a laser is that of a damped second-order system, with a resonance at the relaxation-oscillation frequency $f_R$ and a 3 dB modulation bandwidth of about $1.55\,f_R$ when damping is low. The resonance frequency grows with the square root of the current above threshold,

$$f_R = D\sqrt{I - I_\text{th}},$$

where the D-factor collects the differential gain, confinement factor and active volume. For a DFB with $D = 2$ GHz/√mA:

Doubling the current above threshold raises $f_R$ by $\sqrt{2} = 1.41$, so each increment of bandwidth costs more current, heat and reliability margin. Damping, described by the K-factor, caps the achievable bandwidth, and package parasitics often limit it first. Short cavities with high differential gain, such as strained multi-quantum-well DFBs of a few hundred micrometers, give the highest $D$.

Chirp

Changing the current changes the carrier density, and with it both the gain and the refractive index of the active region. The coupling between the two is the linewidth enhancement factor $\alpha$, typically 2–5 in quantum-well lasers, and it makes the optical frequency follow the power:

$$\Delta\nu(t) = \frac{\alpha}{4\pi}\left[\frac{d\ln P}{dt} + \kappa P\right].$$

The first term is transient chirp, present on the edges of each pulse; the second is adiabatic chirp, a steady frequency difference between the one and zero levels set by the gain-compression constant $\kappa$. The frequency chirp entry works through both and their sign.

Dispersion and reach

In fiber with dispersion $D_\lambda$, a frequency difference $\Delta\nu$ becomes a wavelength difference $\Delta\lambda = \lambda^2\Delta\nu/c$ and a delay difference $D_\lambda L\,\Delta\lambda$. An adiabatic chirp of 10 GHz between the levels, typical of a 10 Gb/s DFB driven with a large modulation swing, is 0.080 nm at 1550 nm. With 17 ps/(nm·km) of standard-fiber chromatic dispersion, the ones and zeros separate by 1.36 ps per kilometer: 27 ps after 20 km, a quarter of the 100 ps slot at 10 Gb/s. Transient chirp adds pulse distortion on top. Uncompensated 1550 nm DML links at 10 Gb/s were therefore limited to tens of kilometers at most, while 1310 nm DMLs, operating near the zero-dispersion wavelength, reach 10 km with little penalty.

Extinction ratio trade-off

A large extinction ratio requires the zero level near threshold, where the photon density is low, $f_R$ falls, turn-on delay and ringing grow, and transient chirp increases. A DML is therefore biased with its zero level well above threshold, accepting an extinction ratio of a few dB in exchange for speed and lower chirp. Higher extinction ratio and low chirp are the province of external modulators.

DML compared with external modulation

An externally modulated laser (EML) integrates a CW DFB with an electro-absorption modulator on one chip; it has lower chirp and a higher extinction ratio, at higher cost and with some optical loss. A Mach-Zehnder modulator can be driven with zero or controlled chirp and supports coherent formats, and is used for long-haul and metro links. DMLs keep the lowest power consumption and cost and remain the choice for short reach: VCSEL links in data centers, 1310 nm DFB links up to about 10 km, and upstream transmitters in passive optical networks.

Common questions

What is the difference between a DML and an EML?

A DML is modulated through its own drive current; an EML runs a laser at constant current and modulates its output in an integrated electro-absorption section. The EML has less chirp and longer reach at 1550 nm; the DML is cheaper and draws less power.

What limits the bandwidth of a directly modulated laser?

The relaxation-oscillation resonance and its damping, together with thermal limits on the bias current and the electrical parasitics of chip and package.

Why do directly modulated lasers chirp?

The refractive index of the gain medium depends on carrier density through the linewidth enhancement factor, so modulating the carriers modulates the optical frequency as well as the power.

References: L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012); G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed. (Wiley, 2021); T. L. Koch and J. E. Bowers, "Nature of wavelength chirping in directly modulated semiconductor lasers," Electron. Lett. 20, 1038 (1984); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).