## 1 Introduction  
A combination of a low power laser oscillator (often referred to as'seed' or'master' laser) and external amplifier is widely used to increase the intensity of a laser beam, while retaining important characteristics of the oscillator. Depending on applications, these characteristics can be either wavelength and linewidth, pulse envelope, other parameters such as beam profile and polarization or any combination thereof. For low gain media, such as Titanium:sapphire (Ti:sa), a single-pass amplifying stage is usually insufficient, so that a resonant or regenerative amplifier stage is more practical. If a narrow spectral linewidth and precise transfer of the wavelength from seed to amplifier output is desired, the laser frequency of the seed needs to be at least partially stabilized to a longitudinal mode of the amplifier cavity or vice versa. Such pulsed narrow linewidth laser sources are desired in many types of atomic and molecular spectroscopic techniques such as laser cooling of antihydrogen [1], measurements of the size of the proton in muonic hydrogen and helium [2, 3] or high resolution resonance ionization spectroscopy for high precision measurements of isotope shifts or hyperfine structure in exotic nuclei. To reduce line-broadening effect Doppler-free two-photon excitation [4, 5], supersonic gas jets [6, 7, 8] or cross-beam geometries [9, 10, 11] have been used. Often a high power laser can even be detrimental in these applications, as power broadening of transition lines may increase the measurement uncertainty. However, many atomic and molecular absorption lines lie in the UV or mid-to-far infrared, not directly accessible with many laser sources. Conversion to these wavelength regions by nonlinear processes in crystals or other nonlinear media requires high laser intensities, especially if convenient single-pass frequency conversion is to be used. Several injection-locked Ti:sapphire amplifiers have been developed for remote water vapor sensing using differential absorption lidar (DIAL) [12; 13; 14]. Other applications include four-wave-mixing [15] as well as coherent Raman spectroscopy and microscopy [16; 17; 18]. Further benefits of injection seeding relevant in these cases are a reduction in timing jitter and intensity fluctuations [19]. In practice the implementation of these laser sources requires two laser cavities and two feedback loops for wavelength control. One loop to stabilize the frequency of the oscillator itself and another to lock the amplifier to the oscillator, also referred to as 'injection-lock'. Injection locking techniques are possible both for continuous wave (CW) as well as pulsed amplifiers. For CW amplifiers the error-signal required to establish the feedback loop is typically generated via modulation and phase-sensitive detection. Dithering of the cavity with piezo (PZ) actuators or higher bandwidth techniques such as Pound-Drever-Hall [20] with electro- or acousto-optic modulators are widely used for this. In pulsed amplifiers the high peak intensity during the pulse can often saturate the photo-detectors used to sample the CW signal and add noise to the system. To circumvent this, additional optical or electronic circuitry to blank the signal during the pulse are often applied [21] or alternative locking schemes, such as 'ramp-hold-fire' [22; 23] or minimization of pulse build-up time [24] can be employed. The need for two separate laser cavities naturally has disadvantages due to the increased complexity, cost and space requirements. Direct generation of narrow-linewidth radiation from a single pulsed laser cavity however is difficult, as each individual laser pulse builds up from an effectively random distribution of photons, so that strong line narrowing via multiple frequency-selective intracavity elements such as etalons, birefringent filters, prisms or gratings is needed. Even if doing so, a truly longitudinal single-mode operation with controllable mode-hop free wavelength tuning is very difficult to achieve.