Spotlight: Injection Seeding

What is Injection Seeding?

Pulsed lasers have a minimum bandwidth which is limited by the Fourier transformation, i.e. given a certain pulse duration the band width of the corresponding pulse can never be smaller than this limit [1]. The Fourier transform limited bandwidth obviously depends on the exact pulse form, but as a rule of thumb one finds that 44 MHz is the bandwidth limit for a 10 ns pulse. The corresponding bandwidths for other pulse lengths can be computed easily based on this rule of thumb (for instance 44 GHz is the bandwidth limit for a 10 ps pulse).
Considering this natural bandwidth limit, it is particularly challenging to produce bandwidth limited laser pulses in the nano-second regime. Those pulses are generally generated by using a Q-switch, which acts as a fast off/on switch of the cavity. The cavity is switched "off" while the inversion in the laser is produced. Once it has built up, the cavity is switched "on" enabling feedback and thus amplification by stimulated emission. Usually, the radiation on a few longitudinal modes of the laser cavity is amplified and emitted leading to non-Forurier transform limited linewidths. A very common technique to achieve Fourier transform limited laser pulses in such a Q-switched alser is injection seeding.
The underlying idea of injection seeding is that one particular longitudinal mode of the slave laser is pre-populated with photons from a master laser. Once the laser action starts, this particular mode has a head start: Rather than from vacuum it starts pre-populated, thus gets more gain, grows more rapidly and finally takes over. More information including mathematical descriptions can be found in an extensive list of publications. As examples, we list the following references [2].
The requirement of this technique is that the slave cavity must be kept in resonance with the photons from the master laser. Several methods exist to achieve this condition.

Cavity Dither Technique

This is one of the standard techniques for stabilization of a cavity. The cavity length is dithered across a resonance and is stabilized by monitoring the transmission of the cavity and generating an error signal, which is used as the feedback on a piezo mounted mirror. An example for a practical implementation of this system can be found in [3].

Minimization of Build-Up Time

Most commercial Nd:YAG systems use this technique. If the pre-population of the cavity mode is optimum, the build-up time of the laser is minimized. The build-up time refers to the time it takes for the laser radiation to build up, i.e. the time from firing the Q-switch until the laser pulse actually exits the cavity. For the build-up time technique this time is constantly monitored and minimized on average. An obvious problem of this technique is that there is no way of measuring the direction of deviation from the optimum cavity length. Thus, the feedback occurs in a random fashion and can only be found rather slowly once a deviation has been detected. In practice one finds that the technique only works reliably for a pre-defined carefully optimized repetition rate of the laser system. In commercial systems, this is between 10 Hz and 100 Hz. Additional information can be taken from the literature [4].

Ramp-Fire Technique

The ramp and fire technique. The length of the cavity is quickly ramped by applying a voltage ramp to the piezo.
When the interference signal has a maximum the Q-switch is fired.

The set-up for this technique is depicted below. The light leakage through mirror M1 is monitored by means of a photo diode, while a piezo stack mounted on mirror M2 ramps the cavity length. The photo diode signal detects an interference signal from the interference of the seed light, which has made one round trip through the cavity and light that leaks out directly. When the interference shows a maximum, the seed laser is in resonance with the cavity and the Q-switch can be fired.
An obvious advantage of this technique is that once the firing of the laser occurs, the resonance is guaranteed. This system therefore works on a single-shot basis and under any environmental condition. A disadvantage is that the exact timing is uncertain, i.e. the laser could fire at any time during the voltage ramp and a synchronization with other events might be impossible. This technique was pioneered by E.S. Fry and co-workers [5].


Ramp-Hold-Fire Technique

The ramp-hold-fire technique is a closely related to the ramp-fire technique. After the ramp is fired and the resonance has been detected, the ramp is stopped and the length of the cavity is held constant until after a pre-defined time after the start of the ramp, the Q-switch is fired. Obvious advantages are that now the laser shot occurs at a fixed time and synchronizing different events becomes easy. A practical problem is that due to the need to hold the ramp, ramping times have to be reduced in order to avoid mechanical ringing in the system [6]


For completeness, we list the self-seeding technique, in which two resonators are coupled. One acts as the small linewidth master cavity and the second as the high-power slave oscillator. More information can be taken from the literature[7].


  1. D.J. Bradley, Methods of generation, in: Ultrashort Light Pulses, Topics of Applied Physics 18, S.L. Shapiro (ed.), Springer Verlag (Heidelberg, 1977), chap. 2, p. 17-81
  2. U. Ganiel, A. Hardy, and D. Traves, Analysis of Injection Locking in Pulsed Dye Laser Systems, IEEE J. of Quant. Elec. QE-12, p. 704 (1976); S. Basu and R.L. Byer, Short Pulse Injection Seeding of Q-Switched Nd:Glass Laser Oscillators - Theory and Experiment, IEEE J. of Quant. Elec. 26, p. 149 (1990); N.P. Barnes and J.C. Barnes, Injection Seeding I: Theory IEEE J. of Quant. Elec. 29, p. 2670 (1993); J.C. Barnes, N.P. Barnes, L.G. Wang, and W. Edwards, Injection Seeding II: Ti:Al2O3 Experiments, IEEE J. of Quant. Elec. 29, p. 2684 (1993)
  3. A. Kasapi, G.Y. Yin, and M. Jain, Pulsed Ti:Sapphire laser seeded off the gain peak, Appl. Opt. 35 p.1999-2004 (1996)
  4. R.L. Schmitt and R.A. Rahn, Diode-laser-pumped Nd:YAG laser injection seeding system, Appl. Opt. 25 pp. 629-633 (1986)
  5. S. Henderson, E.H. Yuen, and E.S. Fry, Opt. Lett. 11, p. 715 (1986); E.S. Fry, Q. Hu, X. Li, Single frequency operation of an injection-seeded Nd-YAG laser in high noise and vibration environments, Appl. Opt. 30 p.1015-1017 (1991).
  6. Th. Walther, M. P. Larsen, and E.S. Fry, Generation of Fourier-transform-limited 35-ns pulses with a ramp-hold-fire seeding technique in a Ti:sapphire laser, Appl. Opt.-LP 40, pp. 3046-3050 (2000)
  7. N.P. Barnes, J.A. Williams, J.C. Barnes, and G.E. Lockard, A self injection locked, Q-switched, Line-Narrowed Ti:Al2O3 laser, IEEE J. of Quant. Elec. 24, p. 1021 (1988) ; D-K. Ko, G. Lim, S.H. Kim, B-H Cha, and J. Lee, Self-seeding in a dual-cavity type pulsed Ti:sapphire laser oscillator, Opt. Lett. 20 p. 710 (1995)


Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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